Pellet velocity versus accuracy test: Part 8
by B.B. Pelletier
Aaron’s winning photo. He’s holding a GSG 92 CO2 BB pistol made by an airsoft manufacturer that now also produces realistic lookalike airguns.
This is an exploration into the theory that high velocity reduces pellet accuracy when it reaches and exceeds the transonic speed region, or about Mach 0.8 to 1.2. We have thus far examined four different .177 pellets at three different speed levels, produced by shooting them in a Whiscombe JW75 spring rifle. Because all pellets have been fired in the same barrel and powered by the same powerplant, the conditions have remained the same, except for their velocities. That was altered by the use of air transfer ports of varying sizes, that passed the compressed air at differing rates.
In this fourth test, I’ll reduce the velocity of the four pellets even more, to as low as I am able to go with this rifle. Then, we’ll have four sets of groups to examine for each of the four pellets. While that isn’t enough testing to prove anything conclusively, it should provide a good indicator of what happens when pellets are both within and outside of the transonic velocity range. The current theory says that pellets are not designed for transonic or supersonic flight and will be less accurate at those speeds than they will at speeds that are less than transonic.
I’ll record the velocities of all four pellets today and then shoot them for accuracy in the next report. We’ll have at least one additional report in which all the results are compared and, to the extent possible, analyzed.
Pause to reflect
Before I start today’s test, I’d like to take a moment to reflect. Although what I’m doing seems normal, but in 50 years it may seem quite exotic. By using a handmade air rifle like the Whiscombe, it’s as if I were shopping for a violin on the streets of Cremona in 1710 and was able to sample the works of Antonio Stradivari as they came fresh from the maker’s hand. Or perhaps more to the point, as though I were able to buy a target rifle with all the supporting equipment directly from Harry Pope. From the accounts I’ve read, shooters who were able to do just that back in Pope’s time revered his rifles as much as today’s airgunners revere a Whiscombe.
What will readers of the future think about our familiar association with an airgun that, by then, will have assumed an elevated cult status? Indeed, it’s almost in that position today. It’s also the perfect tool for conducting the very experiment I’m now reporting, because it can do everything we need while avoiding bias.
Some readers have suggested that just the fact that it’s a Whiscombe brings bias to the table. They say that because this rifle is so well made, it doesn’t necessarily represent most airguns and may be able to tolerate and even ignore the physical constraints we’re testing. I disagree.
The most accurate rifle in the world is still subject to the laws of the physical world. A bullet or pellet in free ballistic flight doesn’t know or care what sent it on its way. If that projectile is unstable for any reason, it’s going to behave just like a top spinning on a flat table. It’ll wobble and move in the direction in which its instability forces.
In fact, because the Whiscombe is so accurate it should be even easier to see those natural laws in action — if they actually work the way we think they do — because the gun doesn’t have all the extraneous “noise” that normally accompanies a spring-piston airgun. By “noise,” I’m referring to the extra vibrations that influence a rougher gun at the moment of, and just after, firing.
The transfer port limiter I installed for this test is the same one that was in the rifle when it was sent to me. So, we should see a large drop in the velocities of all four pellets. Accuracy testing should then prove very interesting.
The first pellet tested was the Beeman Devastator. They averaged just 772 f.p.s. with this transfer port limiter. The spread went from a low of 767 to a high of 779 f.p.s., so 12-foot-seconds from low to high. At the average velocity, they were generating 9.4 foot-pounds. That’s a velocity loss of 200 f.p.s. from the last test, which should do something to the group size.
Next, I tried Crosman Premier lites. This 7.9-grain domed pellet was pretty accurate pellet in the last two tests, but this time the velocity dropped to an average of 732 f.p.s. That’s about 185 f.p.s. slower than last time. It will be very interesting to see what effect, if any, that has on their accuracy. The spread went from 726 to 736 f.p.s. and the muzzle energy was also 9.4 foot-pounds.
Next up were the most accurate pellets thus far — the Beeman Kodiaks. These averaged 658 f.p.s., with a spread from 655 to 661 f.p.s. That’s an extremely tight 6 foot-second difference between the slowest and fastest pellet in the ten-shot string! And they generated 10 foot-pounds on the nose! That’s more than the two lighter pellets, which isn’t supposed to happen in a spring-piston gun. But it’s exactly what happened last time, as well, so there is consistency.
The final pellet I shot was the heavy 16.1-grain Eun Jin dome. This pellet is really too heavy for the powerplant, when it is set at this level, but we want to see what happens to all pellets on all power settings, so we have to test this one, too. They averaged 501 f.p.s. and ranged from 499 to 504 f.p.s., a five foot-second difference. They weren’t too accurate last time, and I expect them to get worse this time. The muzzle energy was 8.98 foot-pounds, which puts it last in terms of power. That remains the same as it has been throughout this test.
This sets us up for the next accuracy test, which should be most interesting, given the great velocity reductions we’re seeing. But I wonder if people will accept the results, knowing that they were obtained with a Whiscombe. As I said in the beginning of this report, all I think the Whiscombe does is give us a clear picture of the results. But we shall see.
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