How does rifling twist rate affect velocity and/or accuracy? Part 10
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, we’ll begin looking at the effects of the rifling twist rate on the accuracy of our test AirForce Talon SS rifle in .22 caliber at 50 yards. If you’re prone to jumping to conclusions before all the data is in, I have to caution you that today’s test will look bad because I’m testing the custom barrel that has the 1:22″ rifling twist. We know from the earlier tests that this barrel was most accurate at 10 meters on power levels zero and 6. Above that power level and also out at 25 yards, the accuracy of this twist rate broke down. So, it would be reasonable to assume that this barrel will give results that are even worse at 50 yards.
That didn’t stop me from trying my hardest to shoot well. I was able to watch each pellet go into the target paper because of the distance, and that was disconcerting when the pellets landed so far from the aim point and from each other. Let’s take a look at how the rifle did.
The day was nearly perfect, as it has to be to get good accuracy from pellets at 50 yards. The air was calm, except for some light breezes from time to time. I was able to work around these breezes and get the results I was after.
I decided not to test the rifle on zero power because of the long distance to the target. Any breeze would have so much time to blow the pellets off course that I felt it wouldn’t prove anything. So, both pellets were shot on power levels 6 and 10. That’s how I’ll test all 3 barrels.
You may remember that this barrel produced velocities that were very close to each other at power levels 6 and 10. With 14.3-grain Crosman Premiers, the respective velocities were 840/854 f.p.s.; and with 15.9-grain JSB Exact Jumbos, the velocities were 817/815 f.p.s. We expect the pellets in this test to go to the same place on the target, and I would expect the two groups for each pellet to be pretty similar in size.
I started with Crosman Premiers and the power set to 10. I did not adjust the scope since completing the 25-yard accuracy test and the center of the group landed about 3.9 inches below the aim point. Ten pellets went into a group that measures 2.04 inches between centers.
Ten Crosman Premiers went into 2.04 inches at 50 yards on power setting 10. The center of this group was about 3.90 inches below the aim point with the scope set for 25 yards. The pellet at the top center is part of another group — not this one. I did account for the full size of the pellet on the left that just clipped the edge of the target paper.
On power setting 6, the center of the group also struck the target 3.90 inches below the aim point. These measurements of the groups are just approximate since the center of each group was difficult to locate precisely. The 10-shot group size on power setting 6 was 2.607 inches between centers. This is slightly larger than the group shot on power setting 10.
JSB Exact Jumbo
Next, it was time to test the JSB Exact Jumbo pellet. I started with power setting 10. The center of the group landed about 4.25 inches below the aim point.
Ten pellets shot on power setting 10 went into a group that measures 2.509 inches between centers. The group is much taller than it is wide.
On power setting 6, the 10-shot group size was 3.222 inches between centers. This group is considerably wider than the group shot on setting 10. Why that is, I have no idea.
As expected, neither pellet did especially well at 50 yards with the 1:22″ twist barrel. They did stay closer together than I expected, however.
The Premiers were more accurate than the JSBs, which parallels what both pellets did at 25 yards.
I don’t see any real evidence of tumbling pellets with either pellet on either power setting, so it’s too simple to say they’re just destabilizing. They’re less accurate but still stable at this distance. There’s probably something profound in that — something like the pellets still fly point-forward, but erratically. I can’t prove anything, yet, but now I have one barrel’s results in the can and it’s time to look at the factory barrel next. And that one has the twist rate that the manufacturer thinks is best for this airgun.
When I pitched the idea for this test as a feature article for Shotgun News, the editor told me he has never seen a test like this before. Neither have I. This may, in fact, be the first time anyone has published the results of testing three rifled barrels of different twist rates in the same gun under the same conditions. It probably has applications in the firearms world as well as for airguns. So, you readers may be in on something that’s being done for the first time.
We still have to test the factory barrel and the 1:12″ twist barrel at 50 yards. As a final report, I’ll summarize the entire test and the lessons I believe it teaches us.