How does rifling twist rate affect velocity and/or accuracy? Part 8
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, I’m presenting the first part of the data collected thus far in this extensive test. My thanks to blog reader Fred DPRoNJ (Democratic People’s Republik of New Jersey) for creating the original spreadsheet for this data. Although I didn’t use his spreadsheet in the publication, I did use it as my worksheet to put this report together. Thanks, Fred!
This is a look at how the twist rate of rifling affects the velocity and accuracy of pellets in an AirForce Talon SS rifle in .22 caliber. We’re testing the same 2 pellets in each of 3 different barrels in the same gun. All 3 barrels are the same 12-inch length. Two of the barrels were custom-made by big bore airgun designer/maker Dennis Quackenbush for this test, and the other barrel is a factory Lothar Walther barrel that comes with the rifle. I chose this airgun for two reasons. First, it allows the quick barrel change that makes this test possible. Second, it has adjustable power so we can vary the power for each pellet we test.
I’m testing the rifle with 2 pellets. The first is the 14.3-grain Crosman Premier that has long been established as one of the most accurate pellets for a .22-caliber Talon SS. The second pellet is the 15.9-grain JSB Exact pellet that I’ve found will sometimes exceed the Premier for accuracy. I don’t believe there’s another pellet in the world that can match either pellet’s performance in my .22-caliber Talon SS.
Velocity was the first thing I tested, and it will be the only thing I report today. There’s too much data in this test to dump it all in a single report, so the accuracy portion of the test will follow tomorrow.
I shot 10 shots with each pellet at each of 3 different power settings on the gun — zero power (as low as the gun will go), power setting 6 and power setting 10. That was done in each of the 3 barrels, so there are a total of 18 outcomes to this test. Those 18 outcomes are shown in the table below.
My Talon SS doesn’t have a power setting scale etched on it, so I put a piece of tape on the side of the gun above the power window. Zero power is with the power-indicating Allen screw head all the way to the left in the window (the same as it would be on a gun that has an index etched on it). I put marks on the tape where power settings 6 and 10 are. I’ve tested many hundreds of these guns over the years that I know where the power screw head has to be, so these settings are quite accurate. The fact that I used the marks for each test (by centering the screw head on the mark) means that everything was identical for each test.
First, I note that in all cases the velocity increased the most between power setting zero and setting 6. The velocity increase from setting 6 to setting 10 was always smaller than the increase from setting zero to setting 6, and that’s irrespective of the twist rate or which pellet was shot.
It’s true that there’s more variation between power setting zero and power setting 6 than between settings 6 and 10, but what you’re seeing here is the slowing down of the rate of velocity increase. That becomes clear in a moment when I discuss the rifle’s maximum velocity potential.
Next, I noted that, as the twist rate slowed (1:22″ is slower than 1:12″), the velocity increased at most power settings with most pellets. There was one instance with the 1:22″ barrel where the 15.9-grain JSB Exact pellet actually went 2 f.p.s. slower at setting 10 than at setting 6, but with all other barrels and all pellets, there was always a velocity increase as the power setting went higher.
Focusing on the 1:22″ barrel for a moment, we see that the velocity increases between setting 6 and setting 10 were not as great as they were in either the factory (1:16″) barrel or the 1:12″ barrel. This suggests what we have suspected all along — that the twist rate of the barrel does slow down the pellet as it gets tighter. And we can see from this test that the phenomenon is most apparent at the lower power settings. At the higher power settings, the differences seem to shrink, indicating that the influence of the power setting is overriding the influence of the twist rate. I believe this is an important finding, and it sets up the next observation, which is that the top velocity of the gun was fairly close for all 3 barrels, regardless of the twist rate. The type of pellet made more difference to the top velocity than the barrel twist rate did.
Specific things we learn from this test
It should be obvious from these results that the Talon SS powerplant has upper limits that cannot be exceeded by forcing more compressed air through the barrel. Better than anything I’ve seen, this illustrates the relationship between barrel length and velocity in a pneumatic airgun. Last week, I had an inquiry from a budding inventor who wanted to know the fastest pellet velocity I had ever witnessed. After I told him what it was, he told me he was working on an airgun design that used compressed gas at 5,000 to 6,000 psi, implying that this would increase the velocity. He told me that he had done extensive research on the internet and was unable to find anything on this topic, despite several of my own reports that address this very thing.
My point is that I get such inquiries all the time from people who are not yet connected to the airgun community and are out there reinventing the wheel. They’re working on a supposition that we all recognize as erroneous — namely that higher reservoir pressure gives higher velocity — but they haven’t gotten far enough into the subject to know that yet. This test serves as a foundation for why we say that barrel length affects velocity in a pneumatic gun.
A second thing that I found interesting is that power setting 6 is very close in performance to power setting 10. In the case of the 1:22″ twist barrel, it’s remarkably close…but it’s close for all three barrels. A prudent airgunner might consider this when setting the power wheel adjustment on his Talon SS, knowing that a lower setting uses less air, yet gives velocity that isn’t that much slower.
A third thing is that the velocity performance of the 1:22″ barrel is so good at power setting 6 that it makes power setting 10 useless. Take that thought just a little farther, and you’ll see that all power settings above setting 10 are pretty much a waste of air in a Talon SS with a 12-inch barrel, regardless of which pellet you use.
If the 1:22″ twist barrel turns out to be accurate, we’ll want to use it in the Talon SS instead of the factory 1:16″ barrel because we’re getting the same performance at power setting 6 that the factory barrel gets at setting 10. On the other hand, there’s not much real velocity difference between the 1:12″ barrel and the factory 1:16″ barrel. So, whichever one is more accurate is the one to go with if the 1:22″ barrel proves not to be as accurate.
That’s all I have to say, but I’m sure that some of our readers will have even more observations to add. Let the discussions begin.