How does rifling twist rate affect velocity and/or accuracy? Part 7
by Tom Gaylord, a.k.a. B.B. Pelletier
This is Part 7 in this lengthy test series that looks at the effects of the rifling twist rate on both velocity and accuracy of a pellet rifle. Today, we’ll look at the 1:22 barrel, which means the pellet will turn once in each 22 inches of barrel it traverses. Of course, the Lothar Walther barrel in the .22-caliber AirForce Talon SS rifle I’m using is only 12 inches long, so the pellet doesn’t even turn one time before it leaves the muzzle, but that twist rate sets the pellet in rotational motion as it flies through the air to its target. The rotational speed will be less than what the 1:16 factory barrel imparts, and much less than the 1:12 barrel we have also tested.
Dennis Quackenbush made the two custom barrels I’m testing against the factory barrel with its 1:16 twist. So far, we’ve tested velocities with 2 different pellets at 3 different power settings for all three barrels (see Parts 2 and 3), and I did a short analysis of those tests in Part 4. Then, we tested the accuracy of the custom 1:12 barrel with both pellets at all 3 power settings at 10 meters, and again at 25 yards. Next, we did the same thing with the factory barrel.
Today, we’ll look at the accuracy of the 1:22 barrel with both pellets at all 3 power setting at 10 meters and again at 25 yards. In the next report, I’ll summarize the entire test to this point for you — comparing all 3 barrels for both power and accuracy. After that, I plan on testing all three barrels for accuracy at 50 yards. At that distance, the pellets will be spreading and accuracy benefits should show up vividly.
On to today’s test — the 1:22 twist-rate barrel.
First up was the 14.3-grain Crosman Premier pellet. I had to remove and remount the scope, and the pellets were now striking to the left and low of the bullseye, but I left it there because where the pellets land doesn’t really matter in this test.
Ten pellets made a group that measures 0.258 inches between centers. Besides being tight, it’s a very round group, indicating the pellet likes this twist rate and power setting.
Next came 15.9-grain JSB Exact pellets on zero power. They also made a round group, but it was larger, at 0.324 inches. This is still a very nice group, but not as nice as the Premier group on the same power setting.
Next, the power was dialed up to 6, and I shot a second group of Premiers. This time, the group was wider than it was high and measured 0.293 inches between centers. That’s smaller than the previous group of JSBs but slightly larger than the Premiers on the zero power setting.
Following that, I shot 10 JSB Exacts on setting 6. They gave a group that is more vertical and measures 0.309 inches between centers.
I noticed at this point in the test that both pellets were loading very easy into the breech. I wouldn’t call them loose — just very easy to load.
It was time to dial the power up to 10 and see what happened. Premiers went first, and 10 of them went into 0.288 inches. That’s just slightly larger than the first 10 on zero power.
And, finally, I shot 10 JSB Exacts at 10 power. They spread out more than expected, giving a group measuring 0.53 inches at 10 meters. That was by far the largest 10-meter group.
What I see here is that Premiers are very stable in the 1:22 barrel. There is little difference in group size at any power setting. JSB Exacts, on the other hand, get progressively worse as the power increases. If we see this much dispersion at 10 meters the difference should be even more visible at 25 yards.
First up at 25 yards was the Crosman Premier with the power set to zero. The 10-shot group landed very low on the target paper, and measured 0.671 inches between centers.
Next, I tried 10 JSB Exacts at the zero setting. They were horrible — making a vertical group measuring 1.949 inches between centers. I won’t shoot this pellet at this power at 50 yards because they would go off the paper!
Next, the power was increased to 6 and Premiers were loaded again. Ten of them made a horizontal group that measures 0.845 inches between centers.
Then it was the JSB pellet’s turn. Ten Exact Jumbos landed in 1.797 inches, which is a little smaller than the group when the power was set to zero. If I try to extend this pellet and power setting out to 50 yards, I’m very likely to get a 7-10-inch group.
Finally it was time to try the pellets on power setting 10. Here they would be traveling their fastest, which means the spin rate would also be highest for this barrel. According to the theory, the groups should get smaller.
Premiers went first, and 10 of them landed in a group measuring 1.082 inches between centers. That’s larger than both groups that went before. Since the velocity increased, the Premiers spread out. Interesting!
Finally, it was time to try the JSB Exact Jumbos on power setting 10. This time the theory did play out as expected, because 10 pellets made a group measuring 1.172 inches between centers. It’s smaller than the group from both of the lower power settings, and those groups decreased in size as the power increased.
Premiers behaved differently than JSB Exact Jumbos in this test. They did not become more accurate as the velocity increased, and I think I can suggest a reason why. JSBs are longer than Premiers. Premiers measure 0.269 inches in length, while JSB Exact Jumbos measure 0.296 inches in length. At their widest, which is the skirt, Premiers are 0.220 inches in diameter, while JSBs are 0.222 inches across. So, JSBs are longer than Premiers, in relation to their diameter, and that makes them harder to stabilize.
That was one of the problems I had with the .22 Hornet centerfire rifle I reported on last week. It shoots its bullets very slow, relative to other .22 centerfires, yet the twist rate is 1:16, where other .22 centerfires are 1:12, or in the very specialized instance of the .223/5.56mm, anywhere from 1:7 to 1:12. That’s why I’ve been writing about these rifles — so we can all gain an appreciation for how twist rates affect accuracy. The .22 Hornet can only do its best with short, fat bullets of relatively light weight. Now, you see the same thing in a pellet rifle.
Today, we see a very dramatic result of how the twist rate affects accuracy. We learned in our test of the smoothbore pellet gun that while a gun may be accurate at 10 meters, it may fall apart at 25 yards. Today, we see that in a rifle that has a very slow twist rate doing the same. If we wanted to use this twist rate, we would need to shoot only very short pellets so they could stabilize. See how it works?
Next, I’ll write up a summary article of the test to this point so we can get a grip on all the data that’s been generated. Of course, it’s all here for you now. All you have to do is go back and look at the results of all the testing to see how the twist rate affects both velocity and accuracy.
Following the summary report, I’ll test all three barrels at 50 yards.
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