Testing the effect of barrel length on a precharged rifle
by Tom Gaylord, a.k.a. B.B. Pelletier
This test was done for blog reader GunFun1, who asked to see what effect barrel length has on velocity. Though it appears simple, this test took 2 days to conduct because of barrel changes and other sundry things. But what was learned far exceeded my hopes, so the effort it took was well worth it.
I tested with an AirForce Talon SS, which has the facility to accept interchangable barrels. All testing was done with the rifle in .22 caliber, which means every barrel used was that caliber. I used the factory-installed 12-inch Lothar Walther barrel, an optional 18-inch Lothar Walther barrel and an optional 24-inch Lothar Walther barrel.
I fired 5 shots at each power level with each barrel. I could have shot more; but since I know the stability of the Talon SS powerplant, it really wasn’t necessary. And fewer shots made the test go faster. After every power change, I fired one shot before testing to settle the valve at the new level. I know that’s necessary for the Talon SS. Other PCPs may have different techniques after making power adjustments — including no warmup shots at all. But the SS requires one shot after every power adjustment. After testing each barrel, I refilled the gun to 3,000 psi. The pressure remaining in the reservoir when I began each fill was about 2,600 psi. So for all shot,s the rifle was running right in the middle of the optimum power curve.
I started each test on the highest power, then dialed back as the test advanced. That meant that when I got back to power settings 6 and zero, the rifle was in the middle of the power curve and was at its most stable condition.
I used only a single type of pellet — the 14.3-grain Crosman Premier. Had I used additional pellets, there would have been different velocities. The relationships we’re looking for are revealed in this one pellet as well as if I’d tested greater numbers of pellets. There was no need to waste shots or air.
I filled the air tank to 3,000 psi, then shot 5 rounds on the highest power, then 5 at power setting 10, then another 5 at power setting 6 and finally the last 5 at power setting zero. After every power adjustment, one dry-fire shot was taken to set the valve at the new power setting.
First, let’s look at what happened with all 3 barrels at the maximum power setting. The 12-inch barrel gave an average 835 f.p.s. The 18-inch barrel averaged 921 f.p.s. and the 24-inch barrel averaged 1024 f.p.s. That’s all with the same powerplant, the same amount of air being used with each shot — in fact, everything was the same except for the length of the barrel. This is a clear demonstration of what a longer barrel can do for a precharged gun.
However, there’s one thing we need to note. The barrel I used for the 12-inch barrel is not the same 12-inch barrel that was used in the previous test of rifling twist rates. If you look back at that test, you’ll see that this same rifle shot a Premier pellet on power setting 10 (854 f.p.s.) faster that it did in this test (827 f.p.s.). The reason is probably due to small differences in the individual barrels — the smoothness and diameters of the bores and how well the backs of the barrels seal against the air valve in the reservoir. So, one 12-inch barrel can be different than another 12-inch barrel — even when they’re both produced by the same manufacturer! That’s worth noting. I used to see this all the time when I tested guns while I worked for AirForce, and now I’m showing you what I used to see. A lot of shooters don’t understand or even believe this can be possible. They think that one barrel must be exactly identical to another barrel of the same specifications made by the same manufacturer.
A second thing to note is the fact that the 12-inch barrel didn’t get much more from the maximum power setting than it did on power setting 10 (835 f.p.s. to 824 f.p.s.). In fact, that held true up through the 24-inch barrel, which tells me that power setting 10 is about as high as I need to go to get the most from this particular rifle’s powerplant. Other PCPs that have adjustable power will behave differently than this, but they’ll all have settings that get the most effective use of their air. Anything more than that is just a waste.
Now, look at power setting 6 across all 3 barrels. The velocity increase as the barrel lengthens is smaller with this power setting than with the higher settings. Also, look at the 12-inch barrel between setting 6 and setting zero. That’s where the bulk of the adjustability for this powerplant is when that barrel is installed. But with both the 18-inch and 24-inch barrels, the useful power adjustment range extends all the way up to power setting 10.
Finally, power setting zero had a surprise. The 12-inch barrel was the fastest of the 3. That can be explained by more friction in the other 2 barrels, but it doesn’t explain why the 18-inch barrel was slowest and the 24-inch barrel was faster. That’s one of those anomalies you sometimes see when you test things like this.
I also want to say that the rifle became quieter with the 18-inch barrel, but got noisier again with the 24-inch barrel. You Talon owners don’t have the shrouded barrel the Talon SS owners have, but apparently your barrels are doing a great job of quieting the shots all by themselves.
Of course, all of this was made possible by the use of a chronograph. No amount of listening to how long it takes the pellet to hit the hickory tree from the back door will give you results like these.
A word from Edith
As many of our readers know, this blog was originally started on the Blogger site/software, and they’re listed in the right-hand column as the Historical Archives. Those blog posts are being moved to this site from Blogger so everything’s in one place. As originally planned, the comments to the old blog would have been lost. Due to the diligence of several of our blog readers who gave me links to help pages that showed how comments could be transferred along with the posts, we’re not losing anything!
The transfer process is almost complete, and the old blogs/comments will soon be available on this site. One caveat: Blogger posts didn’t have categories and tags when this blog first started, and we didn’t start using them when that feature was added later. So, in my spare time (imagine hysterical laughter at this point), I’ll be categorizing the old posts and creating tags that will help you find related items quicker than if you had to do an ordinary word search.