Do-it-yourself project
Gas attack
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, blog reader Vince continues the saga of converting a steel spring rifle to use a gas spring. We last read about this project in Part 2 of I’ve got gas, where he showed us the pitfalls of making such a conversion to a Gamo breakbarrel. Let’s see how he does the second time around with a Crosman rifle.
If you’d like to write a guest post for this blog, please email us.
by Vince
Back when I tried reworking the Crosman gas spring retainer, I discovered that drilling a straight and properly located hole on a round surface is a bit, well… tedious. And hard to do, at least without the proper drilling jig.
Of course, it would be very expedient to use the spring tube itself as a jig. After all, it’s perfect — as long as I can keep from damaging it, that is — because all the holes are obviously already in the right places. Put the retainer in place, pop in the pin and go at it through the existing bolt hole.
Two minor problems became apparent. First, the hole is too large to properly guide a 1/4-inch bit. Second, the edges of the bit might damage the existing hole in the spring tube. But both problems have an obvious, simple and cheap solution: a bushing.
A bushing for under $1.00 from McMaster-Carr.
I got mine from McMaster-Carr (part #2868T44) for less than 70 cents. If you’re feeling rich, you can probably get an equivalent at Home Depot for about $3.00. The important thing is that it has a 1/4-inch inside diameter, a 5/16-inch outside diameter, and that it be made from brass, bronze or steel. Plastic probably isn’t a good idea.
The process is simple — and THIS time I’m doing it on a Crosman rifle instead of a Gamo. No particular reason, I just wanted to show that it works on the Crosman platform as well. Specifically, this is a Crosman Sierra Pro, but mechanically it’s the same as the other Quest variants.
It looks like a Gamo, talks like a Gamo…
I ran into a bit of a problem sliding out the piston — it seems that the scope stop screw that I identified in this picture was binding the piston. Backing it out one turn solves the problem. As expected, the threaded hole in the Crosman gas spring retainer doesn’t align with the one in the spring tube — just like the Gamo.
The Nitro-Piston gas spring retainer…
…and why it doesn’t work.
So, what we’re gonna do is turn it 90 degrees and drill a hole on the other side.
This is where we have to drill.
See that little ledge sticking into the hole? I’m going to grind it out of the way:
Ground a flat spot, just in case.
In retrospect, though, this step may have been unnecessary.
Setting up the jig is about as straightforward as it gets. After installing the gas spring retainer and securing it with the retaining pin, I place the bushing in the hole in the spring tube and start drilling. The steel is pretty soft, so it’s not that difficult.
The bushing sits in the hole and is the jig for drilling. Simple!
But I only drill about half way and for a very good reason. If I keep going like this, I’ll hit something I don’t want to hit. Not a water or gas main, but that retaining pin is very definitely in the path of that drill bit. The solution is to slide the pin almost all the way out (but still engaging the retainer on one side); so when the bit breaks through, the pin won’t be damaged. Drilling the rest of the way thus proves uneventful.
Don’t want to drill through that pin.
Next comes the tapping — M8×1.25 inches, which is very close to 5/16-inch NC. If you don’t have a metric tap, get one. Do NOT try to make the SAE size work. You’ll regret it if you do! But my old and worn tap steadfastly refuses to start because it wants something a tiny bit bigger than 1/4 inch, so I have to bore out the hole to 17/64 inches. That makes all the difference, and a few minutes later I have a properly tapped hole.
Just a smidgen bigger…
…before I can tap the hole.
A quick test-fit shows that everything goes together just as it should.
Success!
As for the rest of the work, it’s a simple matter of cleaning everything out, lubrication and assembly. If you recall, the gas spring got scratched up from rubbing on the piston in my Gamo 220, so I colored those scratches with a Sharpie. That way, if I wind up with more rubbing in the same place, it’ll be readily apparent.
After a good cleaning, everything goes back together just as I described for the Gamo. But don’t forget that little disc that goes into the retainer.
I suspect this may be important.
One thing I sort of glossed over last time is how to get that pin installed. Since the gas spring has all of about 1/8 inch of preload, the pin can be started using a screwdriver to pry the retainer into place.
Prying the retainer to start the pin.
That’s good for getting the pin started. But you won’t be able to get it the rest of the way through because that spring is still pushing the retainer rearward, and the itty-bit of slop in the whole thing means that the holes won’t quite line up on the other side of the tube.
The solution is easy enough. Once it’s started, tap the pin in until it gets to that point. Then, lay the action on its side with the protruding pin downward, and push down on the spring tube while tapping the retainer with a hammer or mallet. The impact of the hammer will make the retainer jump forward just enough to momentarily line up the holes and allow the pin to start coming through. Three or four taps ought to be enough.
Tapping the retainer allows the pin to slide home.
The only minor difference between this Crosman gun and the Gamo is the endcap, which on the Crosman slides inside the tube. It’s a little different from the one that comes installed on the springer:
The gas spring endcap is on the left, the original on the right.
The gas spring version just slides into the rear after it’s all together, and we’re done!
Don’t forget to tighten the scope stop screw.
The action reinstalls in the stock with no mismatched screw holes.
Shooting it demonstrates the same sort of changes in behavior as with the Gamo I converted, only more so — and less so — all at the same time. For one thing, it runs a little hotter than it did in the Gamo. With the same RWS Basic pellets, it did the following:
991 f.p.s.
997 f.p.s.
1014 f.p.s.
1005 f.p.s.
990 f.p.s.
1007 f.p.s.
992 f.p.s.
1002 f.p.s.
1004 f.p.s.
994 f.p.s.
That’s an average of just about 1000 f.p.s., or 15.5 foot-pounds of energy. This represents an improvement of just about 100 f.p.s. over the original Crosman powerplant.
Firing behavior and feel, however, wasn’t as vastly different as it was in the Gamo. The Crosman “sproings” a fair bit less than its Spanish forebearer (the rear guide tends to be a tighter fit); and with a tarred spring, the smoothness of the firing cycle is pretty close to that of the gas spring.
After I returned the Crosman to its original configuration, I was able to examine the gas spring for any damage. Oddly enough, there was some scratching again but nowhere near as bad as the last time and only on the front 1 inch of the cylinder. So, it’s not related to the centering of the gas spring at all. I suspect the end of the cocking link may have been rubbing it.
I remember a while ago a reader asked about the specs of the gas spring, in particular its pressure. I decided to measure that using my high-precision bathroom scale (!) and a Chinese hydraulic press. This was a quick and dirty way to get a ballpark figure. The pressure was almost constant as it was compressed but not quite. It did creep up just a bit, starting at about 130 lbs. and ending in the vicinity of 150. Overall length of the spring is 10.25 inches with a cylinder diameter of 0.715 inches.
And that pretty much wraps up my gas attack. Exactly where does that leave us?
Well, we’ve shown that the gas spring conversion is certainly doable. It’s not as straightforward as I would have liked — buy a few parts and stick ‘em in — but it’s not beyond the realm of the average handyman. The gas spring itself pretty much lives up to its reputation… smoother, somewhat harder to cock for a somewhat elevated power level. The big mechanical advantages — no coils to break, no degradation from being cocked for long periods of time — are already well-known. The main subjective advantage, the smoothness of the firing cycle, all depends on how bad was it to start with. In a 10-year-old Gamo, the improvement is likely to be rather spectacular (especially in an untuned gun), but if the rifle is already a smooth shooter, less is going to be gained. I guess it just comes down to personal preference — whether it’s worth $50 and a couple hours of your time is up to you.
I’ve got gas: Part 2
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, we’ll finish the conversion of a Gamo 220 from steel spring to gas spring, and blog reader Vince gives us a report on the outcome.
If you’d like to write a guest post for this blog, please email us.
Take it away, Vince!
by Vince
When we last saw the Gamo 220, I’d disassembled the powerplant and compared the old parts to the parts I ordered from Crosman. Today, I’ll install those new parts and test the gun for you.
The gun is laying on the bench, ready for assembly. The new piston slides in, followed by the gas spring. Be careful when sliding the piston seal past the end of the cocking slot and tuck the soft seal material away from the sharp edges of the cocking slot so the seal isn’t damaged. A flat-bladed screwdriver works well for this.
The new piston that works with the gas spring is slid into the spring tube. Notice that I’ve lubricated both ends of the new piston with moly grease.
The new gas spring (Nitro Piston) slides in after the piston. The small end of the spring fits into the socket inside the new piston I mentioned in Part 1. No lubrication is required.
The trigger and cocking link go back in (reverse order of removal), and the plain plate gets dropped into the rear spring retainer.
I’m dropping the plain plate into the rear spring retainer.
Now, I’m starting to sweat a bit. You see, I KNOW that the gas spring has a TON of pressure on it even when fully extended (very much unlike a coil spring) — so, how on earth am I gonna compress it enough to reassemble the gun? Oh, well, I’ll cross that bridge when I come to it –which is, well, right about now. After I install the rear retainer, I notice something.
There’s almost no preload on the gas spring
Almost no preload at all! THAT’S right. Because the gas spring is ALWAYS at or near full pressure, there’s plenty of preload pressure as soon as the piston comes off its stop, so very little preload travel is required.
What is preload?
When a conventional coiled steel mainspring is installed in a spring gun, it’s usually longer than the space into which it must fit. It is, therefore, necessary to compress the spring by some amount to get it to fit inside the spring tube. This compression causes the spring to be under pressure even when at rest — this is called preload. If you’ve ever seen a long, empty flatbed trailer on the interstate that looked bowed up in the center because there’s no weight on it, you’ve seen what no preload looks like. It takes several tons of weight just to get that trailer flat again — and much more to make it bow the other way.
Airgun tuners can add spacers that preload the mainspring even more when it’s resting, which causes it to develop greater power when compressed because it’s closer to its maximum potential that exists at the point when all the coils are touching. But gas springs don’t work that way. They’re under full compression (internal gas pressure) when they’re at rest. All cocking the gun does is move the internal piston against the already-compressed gas that’s ready to blast it back when the sear releases it. There’s a very small amount of additional compression of the gas, but it isn’t what makes the gas spring work as well as it does. The gas spring unit is always at full potential — even at rest.
So, this gas spring unit has very little farther to go at this point…under a quarter-inch, in fact. THIS sure makes things easy for me. Pry the retainer forward on one side while starting the pin through the other. [Note: If I used a mainspring compressor, I wouldn't need to pry anything. I would just tighten the compressor until the assembly pin holes lined up, then insert the large crosspin.]
The crosspin will go in, but the hole for the rear spring retainer bolt (that large-headed bolt I removed when I disassembled the powerplant in Part 1) doesn’t line up with the hole in the spring tube. This is a problem.
Immediately, a problem becomes apparent. Look at the hole where the rear spring retainer bolt goes. It’s not lined up with the hole in the tube. There’s approximately a .080″ misalignment here. This ain’t gonna work. My first inclination is to simply elongate the hole. But when I reinstall it, there’s another problem.
There’s a gap between the plate on the spring retaining bolt and the trigger assembly. It won’t support the trigger this way!
The trigger isn’t properly supported by the plate that’s attached to the bolt. Worse, this changes the spacing between the front and rear stock screws and doesn’t allow the action to be reinstalled.
Hmmm. I’m wondering if this is exactly what Crosman (or BAM) had in mind — preclude an easy conversion with existing parts (since the same problem would exist on a normal Quest). That leaves me thinking: Can I just butt the gas spring against the original Gamo spring retainer?
If you look at the picture of the new rear spring retainer above, you’ll see that there’s a small plate that drops into the cup that retains the gas spring cylinder. The cylinder wants to butt up against a flat surface, and the Gamo retainer has a large (approx. 1/2″) hole in it. I need a metal plate to go over it. Wait a minute! I’ve got one right here in my pocket!
A perfect spacer for the new gas spring and it costs — well, about a quarter!
And, so, it gets reassembled. Believe it or not, the whole thing works.
Time to test!
I’ll run through this pretty quickly — the velocity is now up to about 964 f.p.s., which represents a muzzle energy of about 14.5 ft-lbs. Not killer, but obviously a lot better than the detuned gun. Accuracy shouldn’t be changed — or should it? Oftentimes, guys will detune their guns to make them more accurate — or to simply make them easier to shoot. That might have some merit, as I now couldn’t break 0.37 inches at the same range. Not a big difference, and I’m certainly not gonna suggest that the gas spring decreased accuracy. But I don’t think it helped.
So what’s it like to shoot?
First of all, everything anyone ever said about “thunk” vs. “sproing” is absolutely correct. The gun “wumps” with a gas spring, and you can actually feel a kick back into your shoulder. Nothing like a typical centerfire gun, although maybe something like an 1894 shooting low-velocity .38 specials might be comparable. But that’s just a guess.
Cocking the gun is another matter. Effort peaks at about 33 lbs., which isn’t all that high — except for the fact the effort before that peak is certainly a lot higher than with a normal coil spring. This is what we’d expect, of course, with the relatively constant pressure of the gas spring. It isn’t unbearable, but it does take some getting used to.
Back to a coiled steel mainspring
After about 40-50 rounds, I decided it’s time to restore the gun back to original spec. I rummage around my spring box and find a REAL low-mileage Gamo spring, and put it all back together the way God intended it. NOW, I can really get a back-to-back series of impressions.
First, the velocity did drop a smidgen. It’s now down to an average of about 943 f.p.s., or a little under 14 ft-lbs. Second, and despite the tar on the spring and rear guide, we DEFINITELY are sproinging ourselves rather energetically. Lastly, the cocking effort is predictably much milder. Peak effort is down by 5 lbs., and the effort before that peak is even easier. Accuracy is unchanged from the gas spring.
How did my quarter, er, my impromptu gas spring backing plate pan out? Not too well.
The pressure of the gas spring punched a deep divot into the quarter.
The flip side doesn’t look any better.
I flattened it back out with a hammer, and I’m really hoping it’s still legal tender. Anyway, as I sort of expected, the relatively soft quarter didn’t do well. The backing plate really ought to be steel, 0.060 inches (1.5mm or 1/16″), just like the original.
But the bigger problem wasn’t the quarter.
There’s a serious indication of metal-to-metal galling.
There was some serious metal-to-metal contact going on here between the cylinder of the gas spring and the inside of the piston. If you look at the above pictures of the quarter, you’ll see that the indent isn’t centered. The pocket in the original rear spring retainer keeps the spring cylinder right in the middle, and apparently that’s real important because it won’t center itself.
And that’s about it for now. If this is going to work, we need a simple and cost-effective way of keeping the gas spring centered properly without permanently altering the original parts…and do it in a way that the average tinkerer can accomplish on his own. The first thing that comes to mind is to drill and tap a new hole in the new rear spring retainer, opposite of and slightly forward of the existing hole. I tried that, and found (predictably) that getting the hole in just the right spot is a bit difficult without a custom drilling jig.
For now, I’m just going to give it some thought.
WAIT! I JUST GOT AN IDEA….
I’ve got gas! Part 1
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, we’ll have the first part of a guest blog from reader Vince. For those who don’t know him yet, Vince is our “go-to” guy for fixing all sorts of strange vintage airguns. In this post, he tells us the tale of a wild idea he just had to try.
If you’d like to write a guest post for this blog, please email us.
Over to you, Vince!
by Vince
“Nitro” is da bomb, right? I mean, in current usage, “Nitro” anything means hot, fast, powerful and overall bad. This normally benign element sure shows its alter-ego when combined in properly mischievous proportions with oxygen to form nitrous oxide. More fun can be had by mingling it with oxygen, carbon, and hydrogen in various arrangements to come up with nitromethane, nitroglycerin, nitrocellulose or TNT (trinitrotoluene). So, yeah, “Nitro” IS da bomb — in every sense of the word.
Now, when Crosman started producing the Nitro Piston series of air rifles — well, it sends the imagination reeling, doesn’t it? That is, at least, until we come back to reality and realize all they’re doing is using something called a gas spring, which uses pressurized nitrogen to exert pressure on a piston-rod and cylinder assembly. Push the rod in, and it pops right out again, with different degrees of enthusiasm, per the individual design of the particular gas spring.
The advantages of the gas spring in an airgun application are numerous. Because there’s no metal coil spring inside, the gun doesn’t FEEL like there’s a metal coil spring inside. No twang, sproing, buzz or anything else of that nature — and no twisting or torquing reaction as the spring extends and slightly unwinds. Because gas doesn’t fatigue, a properly functioning gas spring will never take a set or get weaker with time. This also means you can leave it cocked for as long as you want with absolutely NO effect on spring life.
What’s not to love?
Well, specifically, those two little words…”properly functioning.” Yes, the gas will never fatigue, but sometimes it leaves home and never comes back. And if the gas spring DOES leak, there’s nothing left to do except go get another one. And almost certainly, it means going back to the manufacturer — and praying that they still have them available.
You see, gas springs are something of a specialty item. They kinda have to be designed around a specific application. There are universal gas springs out there, but the chances of finding one with parameters comparable to your airgun is gonna be difficult. Unlike a coil spring you can’t just get a longer one and cut it to length. You can’t tell what sort of rate a gas spring has (or had) just by measuring things, like you can with a coil spring. I believe you can still get custom coil springs made on a case-by-case basis, but making a gas spring is a bit more involved. Heck, you can make a crude coil spring yourself using a paper clip and a pencil. Sure, it won’t be good for much — but it shows that the basic process for making one is, well, pretty basic.
That’s why I’m not terribly tempted by all these Nitro Piston (of any sort) air rifles that are out there. Will you be able to get it working 50 or 75 years from now? I certainly won’t, cause I’ll probably be gone by then. But that’s beside the point. I’ve got future generations in mind here! I don’t want little Billie- or Betsy-Bob cussin’ out their great granddaddy simply because he bought a gas spring rifle they can’t fix. Heaven knows they’ll probably have enough reason to do that anyway. Why add fuel to the fire?
That’s when I got to thinking. How about gas-springing a spring gun? When the gas stuff craps out, I (or whoever) will easily be able to cram in all the old conventional spring stuff and the gun will be back in business. After examining the Crosman gas guns (for some reason this makes me think of the “fart gun” from Despicable Me), it seemed obvious that they’re based on the ubiquitous Quest platform. That rifle, as we all know (except for those of us who don’t) goes back to the old BAM B18/B19 air rifle. Which, in turn, was a near-clone of the old steel-barreled Gamo series of the time (Shadow, 220, 440, 890, etc.).
Seems to me that a proper pickin’ of replacement parts ought to let ANYONE with a Quest variant (which are as numerous as the stars in the sky) or a Gamo 220 variant to gas ‘em up without giving up long-term serviceability. The best part is that Crosman is generally the most tinkerer-friendly airgun company out there. Not only do they sell parts — ALL parts — for many of their springers, but they’ll sell them to anyone. And they’ll let you keep your first-born, your arms and your legs, and your very soul because their prices are so reasonable.
So it is that I started combing through the parts list for the Storm and the Titan GP air rifles. My suspicion of the close relations was confirmed by Crosman’s designations for these guns — both start with C1K77. I believe C designates the gun family, the 1K means 1000fps, and the 77 points to .177 cal. In any event, the commonality of parts (especially the cocking link, trigger assembly and piston seal) tells me I’m on the right track. Things can get a little tricky, as some parts that are virtually identical might have different part numbers depending on cosmetic details (like lettering). Trusting my own judgment, I came up with a list of 5 parts that I THINK will fill the bill:
Piston………………….BT9M22-03-100B
Gas spring……………BT9M22-00-5
Plain plate…………….BT9M22-00-7
Back spring guide…..BT9M22-00-2
Tube cover…………..BT9M22-00-3
I ordered the parts and waited. As a side note, I told the customer service rep at Crosman what I was trying to do, and he seemed rather interested in my results. I think I’m gonna just send him a link to this blog. As another side note, there’s some commonality between Crosman’s part numbers (which were revamped a couple of years ago) and Stoeger’s. Of course, both guns are made by BAM in China.
A week or so later, the package arrived!
The parts received from Crosman. The camera perspective distorts the gas spring at the hottom. It’s really straight.
Picking a guinea pig is easy. I immediately turn to a tried-and-true, long-time member of my collection: a Gamo 220.
My Gamo 220 was the guinea pig.
The Gamo 220 is a bit of a mule, frankly. All the 1000 fps Gamos of that period used essentially the same powerplant with a 25mm bore, a 100mm stroke, and the same 29 lbs./in. spring. The Shadow, which was the first decent airgun I ever bought, had a nice-to-hold, if utilitarian, synthetic stock and the low-grade rear sight. The significantly more expensive 440 had a nice-looking wood stock with Gamo’s “better” micro-metric rear sight (which, incidentally, is actually inferior to the lower grade one). The upscale 890 was a sightless 440 that came with a scope.
The 220, price-wise, sat in the middle — with a completely unadorned, mud-brown and slippery wood stock with about as much aesthetic appeal as an old shoe. It lacks the utilitarian friendliness of the Shadow and the visual appeal of the 440/890. It really was the least-appealing of the Gamo magnum breakbarrel lineup. Why do I have one?
Because.
As a result, it periodically becomes a test-bed for projects when I don’t want to mar up a NICE gun. Projects like this one.
The first thing I do is baseline the rifle for accuracy and power. In the accuracy department, it didn’t disappoint — a 0.30-inch 5-shot group at 36 feet. That’s about what I remember for the gun. Power-wise, however, was a different kettle of fish. I used RWS Basic pellets for velocity and averaged only 814 fps with a spread of 22 fps. The spread isn’t too bad, but the velocity stinks and represents a measly 10 ft-lbs of power. Oh, well. Frankly, I don’t even remember what guts are in this thing.
The gun comes apart in pretty much standard Gamo/Quest fashion, starting with the three stock-to-action screws. Once the action is out of the stock, you go to the rear of the spring tube, where the big bolt comes out first.
The big bolt comes out first. Then, the pin is drifted out, but only after the end cap is restrained.
The pin is next, but the rear retainer has to be suitably restrained in order to contain the approximately 2 inches and 60 lbs. of mainspring preload typical for these Gamo’s. Or not, as the preload in this gun turned out to be less than an inch. What happened? We’ll find out soon enough.
Next step is removing the trigger, which means the barrel has to be broken open so the trigger can slide backwards a bit.
Break the barrel to put slack in the cocking linkage. The slotted bar that runs back to the trigger is the link for the anti-beartrap device.
Then, the cocking link gets pulled down, freeing it from the piston and allowing you to disengage the anti-beartrap link.
The cocking link can be disengaged from the piston, then the anti-beartrap link disconnects from the cocking linkage.
At this point, the piston just slides out. So — what’s up with that low power and lack of preload?
Wow! That mainspring sure is short. And that rear guide AIN’T Gamo.
Oh, yeah — that’s right. I detuned this thing with a Crosman 500X spring. Incidentally, that spring is part #B12-1-00-4A, for anyone who wants a real pleasant detune on a similar Gamo or a Quest variant. That’s why velocity was so low and why I could cock it with my pinkie! That also explains the lack of preload.
But the guide? Near as I can tell, it was a custom guide I picked up somewhere. It’s dimensionally close enough to the normal Gamo parts that I can be sure it isn’t affecting power.
The spring guide in my 220 (top) and a regular Gamo spring guide.
As a matter of curiosity, I looked at the two pistons side-by-side.
The new piston that goes with the gas spring (bottom) has a slightly shorter stroke than the old piston.
The upper one is the original — and it’s a little shorter, but it appears that, for whatever reason, the gas spring wants a slightly shorter stroke than the coil spring. Also — although it’s hard to tell from the pictures — the inside of the old piston has a flat surface for the top hat, while the newer one has a shallow hole meant to locate the rod end of the gas spring. You will see why this is needed in the next installment.
That’s where we are going to leave this story for now. Vince has the old parts out, and the new parts ready to install. We’ll see what happened in the next installment.
How does rifling twist rate affect velocity and/or accuracy? Part 2
by Tom Gaylord, a.k.a. B.B. Pelletier
Let’s begin our look at the effects of the rifling twist rate on accuracy and velocity. This will be a huge test. I know many of you will want to know THE ANSWER sooner than I get to it. All I can do is ask you to be patient because this has never been documented for the public, if indeed, it has even been done before.
We’re testing the 1:22″ twist barrel that Dennis Quackenbush made for the Talon SS test rifle. I’ll use the velocity figures that I recorded for the factory barrel several months ago in the 10-part Talon SS report. After I’ve tested the 1:10″ twist barrel (in the next report), I’ll also retest the factory barrel following the exact test structure I’m using for both Quackenbush barrels. I know my rifle very well and don’t expect the numbers to be that far off. So, you can accept today’s figures as gospel, but I’ll retest the gun just to make sure.
I followed a fill process that’s very exacting, so each test is the same as all others. I’m not going to bore you with the minutiae, but I discovered while testing the gun on the lowest power setting that the velocity climbed after about 5 shots immediately after a fill, so I refilled the reservoir after each test on low power. On the higher powers, the gun is very stable across the useful fill, so those tests did not all begin at 3,000 psi. They were tested with 2,600 psi to 2,800 psi in the air reservoir — a range where the velocity is extremely regular.
I’m going to use only two pellets initially. Until I learn something about the performance of these barrels, it’s not worth spending endless time running down “facts” that don’t really matter. Later, if the data indicate a need for expanded testing, there will be additional velocity tests with other pellets.
The best way to view the results is when they’re grouped by power setting. Each pellet was tested with the rifle set at three different power settings. Since my gun doesn’t have a scale on the power adjustment window, I put a piece of tape there and marked it for the two higher power settings. The lowest setting is with the power screw indicator as far to the left as the window permits.
Tape marks the two higher power settings. When the screw head is centered on the index mark, the power is correct. When the screw head is as far to the left in the window as it will go, the power is on the lowest setting.
Power setting 0
Factory barrel
On zero power with the factory barrel, 14.3-grain Crosman Premier pellets averaged 486 f.p.s. the range was from 451 to 522 f.p.s. That is an average energy of 7.5 foot-pounds.
On zero power, 15.9-grain JSB Exact pellets averaged 507 f.p.s. The range went from 498 to 521 f.p.s. At the average velocity, this pellet produces 9.08 foot-pounds on this power setting. And the spread is 23 f.p.s.
The velocity spread for both pellets is on the high side, with Premiers being the highest at 71 f.p.s. That tells us the valve is not too stable at the lowest power level and a full fill of air.
1:22 barrel
On zero power with the 1:22 barrel, Crosman Premier pellets averaged 534 f.p.s. The spread went from 499 to 569 f.p.s. — a range of 70 f.p.s. At the average velocity, this pellet produces 9.08 foot-pounds of muzzle energy.
On zero power with the 1:22 barrel, the JSB Exact pellet averaged 521 f.p.s., with a range from 482 to 528 f.p.s. That’s a spread of 46 f.p.s. At the average velocity, this pellet generated 9.59 foot-pounds of energy.
Again, there was a high velocity spread for the Premier pellets, and the JSBs were tighter. With both pellets, the muzzle energy increased with the 1:22″ twist over the factory barrel.
Power setting 6
Factory barrel
On setting 6, the Crosman Premier pellets averaged 787 f.p.s. from the factory barrel. The range was from 775 to 800 f.p.s., so the spread was a tighter 25 f.p.s. At the average velocity, this pellet generated 19.67 foot-pounds of energy.
On the same setting, the JSB Exact pellets averaged 778 f.p.s. with the factory barrel. The range was from 769 to 785 f.p.s., so the spread was 16 f.p.s. At the average velocity, this pellet generated 20.57 foot-pounds of energy.
1:22 barrel
The Crosman Premier pellets averaged 840 f.p.s. from the 1:22 barrel on power setting 6. The range was from 831 to 847 f.p.s., so the spread was a much tighter 16 f.p.s. At the average velocity, this pellet generated 22.41 foot-pounds of energy.
On setting 6, the JSB Exact pellets averaged 817 f.p.s. from the 1:22 barrel. The spread went from 810 to 824 f.p.s. At the average velocity, the energy generated at the muzzle was 23.57 foot-pounds.
Power setting 6 boosted the power a lot. It also stabilized the velocity quite a bit with both pellets. As you can see, the 1:22″ barrel outperformed the factory barrel by quite a lot. This is especially noticeable when you look at the muzzle energy.
Power setting 10
Factory barrel
The Crosman Premier pellets averaged 854 f.p.s. from the factory barrel on power setting 10. The range was from 850 to 860 f.p.s., so the spread was a very tight 10 f.p.s. At the average velocity, this pellet generated 23.16 foot-pounds of energy.
On setting 10, the JSB Exact pellets averaged 823 f.p.s. with the factory barrel. The spread went from 821 to 825 f.p.s., which is just 4 f.p.s. At the average velocity, the energy generated at the muzzle was 23.92 foot-pounds.
1:22 barrel
Crosman Premier pellets averaged 854 f.p.s. from the 1:22 barrel on setting 10 — the identical speed they got with the factory barrel on this setting. The range was from 844 to 863 f.p.s. Although the average was the same as for the factory barrel, the spread was much greater at 19 f.p.s. At the average velocity, this pellet generated 23.16 foot-pounds of energy.
On setting 10, the JSB Exact pellets averaged 815 f.p.s. from the 1:22 barrel. That is LESS than it was on power setting 6. The spread went from 809 to 819 f.p.s. At the average velocity, the energy generated at the muzzle was 23.46 foot-pounds.
Power setting 10 is as high as I ever run my Talon SS. I haver determined that with a 12-inch barrel any setting above this one just wastes air. While the velocities may be a little different with the different twist rates, I believe the general rule will hold that setting 10 is as high as any 12-inch .22-caliber barrel wants to go — at least with the powerplant on my rifle.
The results of this test
As you can see from these results, the gun is wasting air on power setting 10 with the 1:22 twist rate. It is much more efficient on power setting 6. And it does not give up any power to the factory barrel, leading me to wonder if a 1:22″ twist rate might not be a better rate for .22-caliber pellets in the middle power range.
The factory barrel edged out the barrel with a slower twist by getting 23.92 foot-pounds of energy from JSB pellets on power setting 10 compared to 23.57 foot-pounds with the 1:22″ barrel shooting JSB pellets on power setting 6. I don’t know what that says, but there it is.
We’ve learned a little from this test, and we now know there’s so much more to be explored. The results were not as dramatic as some might have anticipated. Many thought the slower 1:22″ twist would have sped up the pellets noticeably, but that didn’t happen. What it did seem to do was make the rifle more efficient in the middle range of power.
It’ll be interesting to see what the 1:12″ barrel does under the same circumstances. After that, I’ll retest the factory barrel at these test settings to verify they’re correct.
How does rifling twist rate affect velocity and/or accuracy? Part 1
by Tom Gaylord, a.k.a. B.B. Pelletier
This is going to be a long report. How long, I can’t say at this time because I’m sure I don’t know everything we’re going to do. Many airgunners have wondered openly how the twist rate of the rifling affects how their guns shoot. But wonder is as far as it’s gone because I’m not aware of any test report that’s ever been written on this subject. Indeed, whenever we get on the subject of twist rates, the equations start flying and we all lean back in our easy chairs while we ponder the implications. But nobody ever seems to do anything concrete to answer the question. That ends today.
What is twist rate and what does it do?
First, let’s all understand what we’re talking about. As a pellet travels down the barrel of an airgun, it’s held by ridges called lands that run along the inside of the barrel. These lands stick up in the barrel and twist in a spiral, engraving the sides of the soft lead pellet and making it spiral as it moves forward. The twist rate is how far the pellet must travel in the bore to make one revolution. A 1-in-10-inch twist rate means that the pellet is turning one complete revolution for each 10 inches of barrel it traverses. That twist rate is written as a ratio — 1:10 inches.
Spinning helps stabilize a pellet in flight. Just as a bullet is made stable by spinning on its axis like a top, a pellet is also stabilized the same way. But pellets of the diabolo design (hollow tail and wasp waist) are also stabilized by the high drag of their tails and the forward weight bias of their design (i.e., more weight in the head than the tail). So, spin is just one of the things that helps stabilize a diabolo pellet in flight, and exactly how much spin it takes is the question we’re discussing. We also want to know what the other effects of the spin rate might be. For instance, does a faster spin slow down the pellet because of greater friction while it’s inside the barrel?
The most common twist rate
When pellet guns were first rifled, they were given the twist rate that was common for the .22 long rifle cartridge at that time, which is one complete turn in 16 inches of travel in the barrel (written as 1:16″). If there was any experimentation with other rates, nothing’s been written about it; so, the pellet gun twist rate has been 1:16″ since the beginning — about 1905. That holds true for all four smallbore calibers (.177, .20, .22 and .25).
I can’t say for certain if other twist rates have ever been used. I hear reports of other rates, such as 1:14 inches, but no proof is ever offered. My thought is that perhaps these other rates are obtained from people incorrectly measuring the twist rate. The simplest way of measuring the twist rate in any rifled gun is to make a mark on a cleaning rod, then pass a wire brush down the bore of the gun on the end of that rod and note how far it goes before the rod makes one complete turn. This method is makeshift, to be sure, but it’s accurate enough for a rough estimate.
I don’t doubt that other twist rates have been used at times, but the makers of those barrels have not made a point of mentioning it in their promotional literature. Twist rate is something airgun writers have elected to ignore over the years, perhaps not finding the subject worth discussion since all airguns seem to have the same rate.
A few of us, though, have wondered what might happen if the twist rate was changed. Since the only way to tell is to test barrels with different twist rates in the same gun, and since nobody makes barrels of different twist rate for airguns, the question has remained unexplored until now. Some time ago, airgun maker Dennis Quackenbush approached me and asked if I would be interested in conducting such a test. I said yes, and we both settled on the Talon SS from AirForce Airguns as the ideal testbed because the barrels can be exchanged so easily.
The experiment
Dennis made two barrels for my Talon SS. Both are .22 caliber, which is also the caliber of the factory Lothar Walther barrel in my gun. He made these barrels from scratch. They’re cut-rifled, which means that each groove is individually cut. Doing it that way, he was able to use a sine bar (a tool used to measure angles) to control the rifling pitch that results in the twist rate.
Talon SS factory barrel (center) is flanked by the two barrels made by Dennis Quackenbush for this test.
Dennis marked each barrel with the twist rate. The top is 1:12 inches and the bottom is 1:22 inches.
Dennis crowned his barrels the same way Lothar Walther crowned theirs.
One barrel he made had a 1:22″ twist rate. That happens to be the twist rate of the .22 short cartridge when a rifle is chambered for that cartridge, alone. The .22 short bullet weighs 29 grains, nominally. If the rifle is chambered for long rifle cartridges, as well, then the 1:16″ twist is used because the longer, heavier 40-grain bullet requires a faster spin.
In the other barrel, he put a 1:12″ twist. That was for no reason other than it’s far enough from 1:16″ and 1:22″ twist rates that there ought to be some differences that can be observed. One curious sidenote to this test is the fact that Aguila makes a special subsonic .22 long rifle cartridge that has a 60-grain bullet. To stabilize that bullet that leaves the muzzle at around 900 f.p.s., a rifle has to have a 1:10″ twist rate. There are special barrels made for the Ruger 10/22 rifle for just that round. Like the Talon SS, the 10/22 has a barrel that’s quick and easy to change.
The most common method of rifling barrels these days is button rifling, in which a hardened “button” is either pushed or pulled through the bore, cutting all the grooves at the same time (actually, it doesn’t cut the metal so much as it “irons” the steel into the desired shape). The button must be made for a single twist rate, taking into account the thickness of the barrel walls and the type of steel in the barrel because the steel springs back a little after the button has passed through. So, the spring rate of the barrel steel must be controlled by the size and shape of the button, as well as the type and thickness of barrel steel, itself. Using a button is a very fast way to rifle many barrels, but it limits you to just one twist rate per button.
Cut rifling is therefore a slower process but does give the barrel maker more flexibility over the type of barrel he makes. Dennis didn’t put a choke into his barrels because we aren’t interested in their ultimate accuracy. But the factory Lothar Walther barrel is choked. So, this will not be a test that pits the accuracy of the Quackenbush barrels against the Lothar Walther barrel. We’ll be examining accuracy potential, but only so far as one twist rate seems to have an advantage over the other with a given pellet at the same power setting. If something interesting pops up, we may wish to explore it further with other barrels in the future.
AirForce helped, too
Dennis contacted John McCaslin of AirForce Airguns for some of the critical dimensions of the barrels. John shared these with him and also provided the bushings for the barrels he made. Even though they were made 12 years after my SS was made, these bushings fit my rifle perfectly.
Test objective 1
The first test objective is to determine the effect of a different rifling twist rate on the accuracy of various pellets at various velocities with barrels of different twist rates. All three barrels will be considered, but we’re really interested in the results of the two barrels supplied by Quackenbush. I can’t say that the factory barrel will be used as a control because it’s made differently than the two barrels Dennis has made (different rifling method and it’s choked). But the data will be included in the test report simply because it exists and may be of interest at some point. Any poential accuracy differences will be noted.
I’ll conduct this test at 25 yards and again at 50 yards. That will tell us how the different twist rates perform at different distances.
Test objective 2
Another factor that airgunners have talked about for a long time is the effect of twist rate on velocity. This discussion has been limited to big bore airguns because, as I’ve noted, all smallbores have the same twist rate. The popular theory is that a faster twist rate will result in a slower bullet velocity when everything else is the same. I’ll test the barrels at different power settings for each pellet I test.
What we’ll get from this test is a broad look at how the twist rate of a rifled barrel impacts (or doesn’t impact) the overall performance of pellets in a gun whose baseline performance we already know very well. I’ve thought long and hard about what would be the best way to conduct the test. Do I test the velocity first and then the accuracy? Or do I turn it around and test accuracy before velocity? I have a plan in mind, but I’d like your input before I start the testing. Remember that I already have a lot of good data on this rifle using the factory barrel.
Summary
As far as I know, an experiment like this has never been published before. Perhaps it’s been done, or perhaps parts of it have been done. If so, they didn’t put it in print. Neither Dennis nor I know how this will turn out. As he says, we’re pushing back the boundaries of ignorance in airguns.
I’d like to thank Dennis Quackenbush in advance for the work he’s done to make this test possible. I also want to thank AirForce Airguns for their part in what we’re about to do.
What can you do?
by Tom Gaylord, a.k.a. B.B. Pelletier
Announcement: Charles Spillman is this week’s winner of Pyramyd Air’s Big Shot of the Week on their airgun facebook page. He’ll receive a $50 Pyramyd Air gift card. Congratulations!
Charles Spillman submitted this week’s winning photo for BSOTW.
Just a reminder that the Roanoke airgun show is next Friday and Saturday — a week from today! I hope to see some of you there.
As immersed as I am in airguns, it’s hard to surprise me these days. But that wasn’t always the case. I remember the time when I shot springers and CO2 guns but avoided precharged guns out of the fear of handling highly compressed air. I saw the movie Jaws and watched the great white shark blow up when the scuba tank let go!
So, I have a frame of reference for the newer airgunners — the guys who may have been shooters for a long time and have now decided for whatever reason to check out airguns. If I write for anyone, I write for them and also for the brand-new shooter.
And I hate jargon. Even though I use it too much, I know how confusing it can be to try to follow a story when the author keeps dropping acronyms and slang terms in your path. I could say PCP, and sometimes I do, but I try to say precharged pneumatic first. And I say silencer, when others attempt to skirt the issue with terms like Lead Dust Collector and Decibel Reduction Device.
Keeping a fresh outlook
One exercise that keeps me fresh is examining and using various firearms (I try never to say “real guns”). Not only does this keep my own perspective fresh and curious — it also gives me a busman’s holiday from airguns, when things become too much the same.
Several years ago, I decided to write a long series of articles for Shotgun News about the Ruger 10/22 .22 rimfire carbine. Why that gun? Because I’d never owned one before and had only shot one a few times. It would appear as new and confusing to me as a Benjamin Trail NP XL appears to a new airgunner. I called my five-part series, “What can you do with a 10/22?”
The Ruger 10/22 is an iconic .22 rimfire. It is to .22s what ARs are to centerfires.
Like any shooter who reads, I had read a lot about the 10/22 — or at least it seemed as though I had. In my mind, it was a superior .22 semiauto that was highly accurate, infinitely reliable and utterly desirable. So, I asked for and received one for Christmas. Just like any airgun, I immediately took it to the range to see what it would do. I was prepared for the best — and got the worst! My 10/22 shot 2-inch, 10-shot groups at 50 yards, on average, with just a couple getting close to the 1.5-inch range. Bummer! I’d owned dozens of .22s that were more accurate. Oh, well, nowhere to go but up!
Just like an airgun, the next step was to tune the rifle. But tuning a .22 means machine time, plus I knew absolutely nothing about the model, so I sent it off to a place in Connecticut that lightened the trigger, installed a trigger stop, jewelled the bolt, drilled a cleaning hole in the rear of the action so the barrel can be cleaned from the breech, rechambered the barrel with a match chamber and installed an extended magazine release that looks suspiciously like a thumbscrew!
Yes, they simply drilled a hole through the back of the aluminum receiver so a cleaning rod could pass through, once the bolt is removed. It’s a great idea!
A simple thumbscrew extends the magazine release so human fingers can operate it. For shame, Ruger!
What I got back was a different rifle. The trigger was now very good (Ruger should be ashamed of the trigger they put in that rifle!), the barrel could be cleaned from the breech, thus preserving the crown, the magazine now pops out fairly easily (another point of shame for Ruger) and — best of all — the rifle was much more accurate. Ten-shot groups of one inch were not uncommon.
Does this look familiar? I tested the Ruger at 50 yards, just like I would an air rifle.
Well, the ulterior motive for buying the 10/22 was so I could buy and test a legal silencer, but the drill to get one occupied more than a year of my time and finally a personal appeal to the head of the branch that approves such requests. I needed something to do in the meantime. My friend Mac donated one of those electric guitar-looking custom stocks to the project, and I bought a Butler Creek bull barrel to see if there would be any accuracy difference…and there was a huge difference! The new custom 10/22 now shot 10-shot groups of less than three-quarters of an inch, with a couple that were under 0.60 inches between centers.
You’re looking at the same rifle in two versions. At the top is the custom stock and bull barrel on the action. Underneath is the standard rifle with the custom trigger features Photoshopped out.
And all it took was spending a further $500 (factoring in the custom work, plus the cost of the custom stock if I had bought it and the bull barrel) to get this $130 semiauto to shoot! Boy, did I ever feel like a new airgunner!
Then, Mac proposed a test of my whomptydoodle custom 10/22 against a box-stock Ruger 10/22 Target — a rifle that Ruger makes and sells for about $450 (at the time the test was being conducted). I said sure, and another huge test was run. At the end of that one, I knew that the factory Ruger 10/22 Target was slightly better than my custom gun, though my rifle has the better trigger. I was kinda like the guy who buys a Beeman HW 97K and then spends $500 getting it tuned, only to discover that the TX200 Mark III was better in the first place!
This is what $450 bought. A Ruger 10/22 Target that out-shot my custom rifle at 50 yards.
By this time, the silencer had come in and I had to reconfigure the rifle with the factory barrel, because the silencer adapter Dennis Quackenbush supplied was made to fit it. I found in that test that a silenced .22 rimfire isn’t as quiet as everyone imagines. Also, the accuracy remains about the same with or without the silencer attached.
And the entire drill was to acquire this legal silencer so I could report on it. It works, but not as well as most people think.
Along the way, I sort of got a brief reputation among the Shotgun News advertisers as a 10/22 guy, so Magnum Research let me test both their .22 rimfire and their .22 Magnum semiautos that are built on the Ruger pattern. The long rifle version is incredibly light, and the magnum gun was a tackdriver. If only I had the money to buy it at the end of the test!
Boy, was this Magnum Research .22 Magnum a tackdriver!
Then, I tested another 10/22 wannabe (the Rhineland R22) and that was the extent of my exploration into this unfamiliar realm. I knew that Volquartsen (a maker of aftermarket upgrade parts for the 10/22) makes complete guns that are no doubt the best that money can buy, but I never sampled them.
This Rhineland R22 doesn’t look like a 10/22, but it is on the inside. This one was chambered for .17 HM2.
But in all of this, I learned some fundamental lessons. First, with all the hype, the Ruger 10/22 is a pretty standard firearm…and other semiautos, like the Marlin model 60, for instance, are just as good. What makes the Ruger stand out are all the aftermarket accessories and all the talk — most of which is just talk and not to be believed. I learned that you do get what you pay for. By spending a little more money up front, you can save a lot more down the road.
Just FYI — this little trip down memory lane originally occupied seven feature articles, taking over 30,000 words and 130 pictures. I mention that because all the detail was omitted for today’s report.
Bending airgun barrels: Part 5
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, I’ll apply what I learned about bending an airgun barrel to a real problem. As I said in several earlier posts, I have a BSF S70 breakbarrel that came to me with a peep sight installed but no open sights. BSF rifles aren’t common in the U.S., so finding a correct rear sight would take some time; but more importantly, I like the peep sight that’s on the gun. It was one that Air Rifle Headquarters sold as an optional sight, but the owner of this rifle removed the original sights and didn’t replace them when he sold the gun. So, all I have is the peep.
When I tried to sight in the rifle, I discovered that it was shooting 2-3/4-inches high at 10 meters, which would put it even higher at 25 yards. Either the front sight had to be raised (because the peep was adjusted as low as it will go), or the barrel had to be bent. Since the front sight is dovetailed into the barrel, I decided to bend the barrel. That was two years ago. Since then I’ve been researching ways to bend barrels and thinking about the equipment needed to do the job.
This report has already documented the simple fixture I constructed, and you can read how it worked in the earlier parts. Now, I’m going to bend the barrel of this pristine collectible air rifle so I can shoot it and hit what I’m aiming at.
The barrel-bending fixture is quite simple to set up. It uses a c-clamp to apply steady pressure on the barrel and a ratchet wrench turns the clamp slowly, so the bend is under complete control.
This was the final bend. This barrel is softer than the first one, but it springs back when the tension is released.
The S70 didn’t fit the fixture the same as the first rifle did. I had to make some adjustments, but there’s enough flexibility built in to allow that. As I started the bend, I noticed right away that this barrel was bending much easier than the other one had.
Metallurgy
Remember what I said about metallurgy of the various airgun barrels? I said that you should treat each new barrel as though you were bending a barrel for the first time. That was good advice for this job, because the S70 required a lot less pressure to bend. So I went slow.
This is how the BSF S70 was shooting before I bent the barrel. The point of impact is about 2-3/4-inches above the point of aim at 10 meters.
The beauty of this fixture is that you can take the rifle out and shoot it after each bend. So that’s what I did. Let me show you the first target, which explains how the work went. I shot the same JSB Exact 8.4-grain pellets as were used when the gun was first fired.
This target documents all the work that was done. It is explained in the text below. The group below the bull is the final 5 shots, 3 of which are in one hole under the number 5. The one at the lower left was fired before I was ready but didn’t bother me.
I knew the rifle was shooting high, and I had the former target that told me how high. So, I first bent the barrel a little then shot the rifle to see what had happened. The first shot after the first bend was still high, but the point of impact had dropped over a quarter-inch. That told me the S70 barrel was bending very easy.
I bent the barrel a second time and put another pellet in the same hole as the first. Then, I bent it once again (three bends so far), and the third shot was the same height but to the right of the first two. Obviously, I had to be more aggressive.
The fourth bend was more aggressive, and the shot that followed dropped into the black bullseye for the first time. Next, I bent the barrel more aggressively, again, and the shot dropped to almost the middle of the red center.
Bend six was even a little more aggressive, and the shot dropped a little more and went to the right. Bend seven was about the same as six, and that shot went into the same hole as shot six had.
Now I knew the barrel needed a lot more pressure to go as far as I wanted. So, bend eight was the most aggressive of all. It’s the one pictured in the photo above. The shot that followed was below the aim point for the first time, which was what I was after. Four more shots were in the same area, though one of them went off before I was ready and struck the target low and to the left. From this result, I knew I was done bending the barrel.
It was time to sight in the rifle with the adjustments on the Williams peep sight that was on the gun. This was the first time since that rifle first got the peep sight back in the 1970s that it’s been able to shoot to the point of aim at 10 meters!
This is the sight-in target. You can see that a lot of upward adjustment was required to get the rifle shooting to the center of the bull at 10 meters.
The sight-in took a long time because the peep sight adjusts in very fine increments. But I managed to walk the pellets up the target until the final one was at the correct height. Now it was time to verify that the gun still shot well. I have the original target for comparison.
This target confirmed that the rifle still shot well. The group made before bending the barrel measures 0.532 inches between centers. This 10-shot group measures 0.506 inches between centers.
As you can see, the rifle shoots as well as before. All that’s been done is adjust the point of impact so the sights can be used at close range.
The Williams sight has a number of screws that are used to lock the sight in position, once it has been sighted in. They all have to be loose to adjust the sight; and when you tighten them at the end, they can make the POI move just a little. You have to check your work after you think the sight is locked down.
So, the fixture I made works for real-world applications, too. It doesn’t take up much space, and I’ll keep it around for any jobs that might arise in the future.
Bending an airgun barrel isn’t something to do lightly; but if you have to do it, it’s nice to know the job can be done with a minimum of tools and time. Today’s project took a total of 45 minutes, which includes bending, shooting and sighting-in the gun at the end of the job.
