How does rifling twist rate affect velocity and/or accuracy? Part 13
by Tom Gaylord, a.k.a. B.B. Pelletier
This is the summary report in this series. I’ll give you my thoughts on how this test went, and I expect you to comment, as well.
Three barrels were used in this test. One was the factory barrel that comes with the .22-caliber AirForce Talon SS. It’s a 12-inch Lothar Walther barrel that has a choke of about a half-thousandth inch reduction in the bore diameter for the final 2 inches of length. That makes all the pellets of uniform size as they leave the muzzle, and it may potentially stop any in-bore wobble. This barrel has the standard airgun twist rate of 1-turn-in-16-inches of bore travel, written as 1:16″.
The other 2 barrels were made by Dennis Quackenbush. Neither barrel is choked. One is a 1:12″ twist; the other is a 1:22″ twist. They’re also about 12-inches long and are held in the gun by AirForce Talon SS barrel bushings. Several comments have suggested that because these barrels are different than the Lothar Walther barrel, this test is somehow not fair. But the results of all the shooting prove otherwise. Sure, there are variations from barrel to barrel, depending on the power used and which pellet was shot. But the results are so close between all 3 barrels that whatever differences there might be are overridden by the similarities. In other words, I’m suggesting that if Lothar Walther had made all 3 barrels, there would be similar differences.
The 3 barrels used in the test. Factory barrel in the middle.
I believe the twist rates are what drive the results. We weren’t searching for the most accurate barrel in this test. We were looking for behavior changes as conditions were changed. And we got that.
The first thing that was tested was velocity. Both pellets — the 14.3-grain Crosman Premiers and the 15.9-grain JSB Exact Jumbo were shot from all 3 barrels at each of 3 predetermined power settings. These settings were marked on the gun so they were kept constant throughout the test.
The power settings were the power indicator screw all the way to the left (the lowest setting), and the power screw centered on each mark (settings 6 and 10).
I reviewed the velocity for you in Part 8. Here’s a summary of that report.
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.
What you’re seeing here is the slowing down of the rate of velocity increase as the air flow increases. That’ll become clear in a moment when I discuss the rifle’s maximum velocity potential.
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 when the JSB Exact pellet actually went 2 f.p.s. slower at setting 10 than at setting 6; but with all other barrels and 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.
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. This illustrates the relationship between barrel length and velocity in a pneumatic airgun.
A second thing 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.
Next, I tested all 3 barrels with both pellets shot at all 3 power levels at 10 meters (11 yards) and 25 yards. Following that, I tested all 3 barrels and both pellets, again, at 50 yards, only I didn’t use the zero power setting. This was where my eyes were opened regarding the effects of twist rate.
I analyzed the accuracy in 2 different reports. One (Part 9) was the 10-meter and 25-yard accuracy and the other (Part 12) was the 50-yard accuracy, alone. Now, with the table above we can combine these results and analyze all the accuracy data together.
The first observation I’ll make is that at 10 meters, I got 10-shot groups that ranged from as small as 0.092 inches to as large as 0.578 inches. The factory barrel gave the best results with the JSB pellet; but with the Premier, it was the 1:22-inch twist that did the best. Curiously, that pellet and twist rate didn’t seem to change that much as the power was increased (at 10 meters). With all other barrels and pellets, the group size did change a lot as the power changed.
It’s too simple to say the factory barrel with the 1:16-inch twist rate is the best; but of the 3 twist rates in this test, it certainly is the most flexible across the board. However, you’ll notice that the 1:12-inch twist barrel did shoot the best single group (with JSB Exact pellets) at 50 yards. That group is so close to the Crosman Premier group shot by the 1:16-inch barrel that I can’t call a clear winner — BUT — here is what I CAN say. The Quackenbush 1:12-inch twist barrel is certainly capable of shooting 50-yard groups at least as tight as those shot by the Lother Walther barrel; and in my mind, that puts the barrel-equivalency question to rest.
Another observation is that the 1:22-inch twist barrel was just as good at 10 meters as the other 2 barrels, in general, but look at how the groups opened at 50 yards! That says something very strong about the relationship of the twist rate to accuracy. And it also brings up a second observation.
Premier pellets and JSB pellets performed differently throughout this test. Just look at the 50-yard results for Premiers and JSBs with the 1:12-inch twist barrel, and you’ll see what I mean. This is one more bit of evidence that barrels have preferences for certain pellets.
This will be my final remark in this series of reports, and it does not come from the data collected in this test but from the 5-part test of the Diana model 25 smoothbore. In that test, we saw that the smoothbore was able to place 10 JSB Exact RS pellets into a group measuring 0.337 inches at 10 meters. However, at 25 yards, the same pellet loaded the same way made a group that measured 3.168 inches. That difference tells us clearly that spin and not aerodynamic drag is the main key to pellet accuracy. I think we now see that twist rates do matter a lot, and the standard rate is the best all-around rate for now.
At 10 meters, 10 JSB Exact RS pellets made this 0.337-inch group.
At 25 yards, the same JWB-Exact-RS pellets, seated to the same depth, made this 3.168-inch group. They are clearly not accurate after 10 meters.