How does rifling twist rate affect velocity and/or accuracy: Part 12
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, I’ll report on the final test in this series. This isn’t the final report — just the final test, which is the barrel with the 1:12″ twist, shooting at 50 yards. Get ready to be surprised. I know I was!
I did this test on the same perfect day as the factory barrel that was reported last week in Part 10, and the weather was perfect most of the time. From time to time, there was a very slight breeze that I waited out before shooting. The shooting conditions were as good as they get.
I used the same two pellets we’ve been shooting all along, and they were shot at power settings 6 and 10…just like the other 2 barrels that went before. The gun was shot while resting on a sandbag that’s very stable. When the tank was filled or the power was changed, I always shot one shot to settle the valve. Experience tells me that’s all that’s needed.
The tank was filled to 3,000 psi.
Power setting 10
I first shot both pellets on power setting 10. And 14.3-grain Crosman Premiers were hitting low and to the left. One of them only nicked the target paper, so I photographed the target before taking it off the backer paper, so you could see the complete group.
Here are the two 10-shot groups of Premiers. Notice that they’re hitting to the right of the aim point, which is the center of the bull they touch. The group shot on power setting 10 is at the top, and it shows why I like to use backer paper when shooting at 50 yards.
Then it was time to test the 15.9-grain JSB Exact Jumbo 15.9-grain pellet on power setting 10. This is where the surprise comes! Ten pellets made a 1.259-inch group! If you check back with the results the factory barrel gave, you’ll see that this group is very close to the best group made by the factory barrel (1.153″ for 10 shots with the same JSB pellet on power setting 6), and it’s equal to the group that was shot on power setting 10 (1.283″). This addresses a question many of have had from the beginning of this test — namely, are the Quackenbush barrels equal to the Lothar Walther barrel?
With so little data, it’s impossible to say if these two barrels are exactly as good, but what we now can say is that one of the Quackenbush barrels gave some groups that are at least equivalent to those from the factory barrel. The difference is so small that it might be due to the twist rate rather than the quality of the barrels. That was the position I took at the start of the test, and I think this demonstrates it was valid.
Power setting 6
Next, it was time to test the 1:12″ barrel on power setting 6. Ten Premiers went into 2.234 inches, which is only slightly smaller than the same pellet on power setting 10. As before, the pellet stuck the target low and to the right.
It’s official — the 1:12″ barrel does not like Crosman Premiers out at 50 yards. But that wasn’t the only pellet in this test.
With JSB Exact Jumbo 15.9-grain domes, the barrel did nearly as good on power setting 6 as it did on setting 10. Ten pellets made a 1.363-inch group. Like the Premiers, the JSBs also performed about the same on setting 6 as on setting 10. But that’s not the real lesson. The real thing to note is that the 1:12″ twist barrel was not as good at 25 yards as the factory barrel, yet at 50 yards it almost caught up. In other words, the accuracy of the factory barrel degraded faster as the distance increased than the barrel with the faster twist.
What has been learned?
This is not the final report. I’ll add these results to the previous summary report given in Part 9, and we’ll be able to see all 3 barrels at all 3 distances with both pellets at all 3 power levels. But if I had to give a quick analysis, I’d say the 1:12″ twist barrel surprised me at 50 yards by being better than I expected. At least it was better with the JSB pellets.
And that fact alone — that a barrel can be so much better with one pellet than with another — is good to know. This test has demonstrated that principle beyond all doubt.
A lot more testing needs to be done to thoroughly see all the relationships, but I’ll tell you what I know in the next report, which will be the final report for this series. I think we can advance our knowledge of how pellets perform by quite a bit by combining the results of this lengthy test, the smoothbore test and the pellet velocity versus accuracy test. We’ve been exploring this theme for nearly 2 full years now, and I think we’ve learned a lot!