Pellet velocity versus accuracy test: Part 6
by B.B. Pelletier
For the benefit of readers who have landed on this article first, this is the sixth test in a series of reports designed to test how velocity affects accuracy. I’m using a Whiscombe JW75 breakbarrel/underlever rifle with a .177-caliber barrel installed. That way the same powerplant is being used for each test. I’m controlling the power of the gun by the use of different air transfer port limiter screws that allow less and less air to past through.
The Whiscombe rifle uses dual opposed pistons that come together to compress the air when the gun is fired. The rifle has no recoil and just a minor impulse that can be felt — yet it’s one of the most powerful spring-piston air rifles ever made. My rifle can produce over 30 foot-pounds in .25 caliber.
Four pellets were chosen at the start of this test and have been used in each velocity and accuracy test that’s been conducted. Two of them were supersonic in the first test and one remained supersonic in test two, while the other was still in the transonic region where wind buffeting occurs. It’s testing exactly what it was designed to.
Today, I installed a smaller transfer port limiter to slow down the four test pellets even more. This was the first time all four pellets were safely below the sound barrier, and the shooting was uniformly quiet. You probably should read the first five reports at the links provided above to understand all that’s happening.
This 7.1-grain lead hollowpoint hunting pellet has been the biggest surprise of this entire series. Normally, these types of special pellets are designed for maximum mushrooming and are not that accurate — at least not in my experience. But the Beeman Devastator has proven to be the exception. From the start, when it was leaving the muzzle at an average 1,216 f.p.s., it was accurate. So far, we’ve conducted only two accuracy tests, so the results of this one should prove interesting. As I said, this was the first time this pellet has left the muzzle at subsonic speed.
The average velocity was 973 f.p.s., with a low of 967 and a high of 977. That’s only a 10 foot-second difference between the slowest and fastest pellet, which I think is amazing. Usually, a pellet this light will also have a much larger total velocity spread over a 10-shot string. At the average velocity, this pellet is cranking out 14.93 foot-pounds of energy at the muzzle. That’s down from the initial 23.32 foot-pounds it was making when there was no transfer port limiter in the gun.
Can you guess what this pellet will do in the accuracy test? I think it’ll be even more accurate than in the past. But who knows?
Crosman Premier lites
Crosman Premier lites were next. Initially, they were leaving the muzzle at 1,134 f.p.s. when there was no transfer port limiter in the gun. Today, they averaged 915 f.p.s. and the spread went from 911 to 919, for a super-tight 8 foot-second difference. At the average velocity, these pellets were producing 14.69 foot-pounds at the muzzle.
Any guesses as to what this pellet will do in the accuracy test? The difference between the first and second accuracy tests was very small, but we’ve finally gotten the velocity down out of the transonic region, where all the buffeting is. They didn’t break the sound barrier the last time; but at an average 1,057 f.p.s., they were well into the transonic range. There could be a happy surprise from the Premiers.
Beeman Kodiaks averaged 819 f.p.s. with this transfer port limiter. The spread went from 817 to 824 for just a seven foot-second total variation across 10 shots. The muzzle energy was 15.2 foot-pounds. Notice that this heavy pellet is producing more energy than the two pellets that are lighter — something that’s not supposed to happen with a spring-opiston system.
I need to interject a word on Kodiaks. For some reason, their weights have varied widely over the past couple years. Where they had weighed 10.6 grains for several decades, someone decided to reduce the weight to 10.2 grains in 2010. Those are the pellets I have. But now I notice that the weight is back up to 10.4 grains. Obviously, someone is adjusting the weight of this pellet that was once considered one of the three most accurate .177 pellets on the market.
Kodiaks have been the most accurate pellets in both the previous accuracy tests (after I started using the scope level), but I don’t know if they’ll continue that trend at this new lower velocity.
Eun Jin domes
The 16.1-grain Eun Jin dome is really too heavy for the Whiscombe in its most powerful form. Certainly as the power is reduced, they become even less appropriate. In this test they averaged 618 f.p.s., and the spread went from 615 to 624, for an 11 foot-second total. At the average velocity, these super-heavyweights are producing 13.7 foot-pounds at the muzzle.
They produced the worst group last time, opening up from the group they had made with no transfer port limiter installed; and I predict this trend will continue in the next accuracy test. These are really best in powerful PCP guns — not in spring-piston guns.
Next, I’ll test the accuracy of these four pellets at 25 yards. I’ll use the same careful technique that I’ve been using so far to keep the results as free from bias as possible, so this should give us a good look at how velocity really affects accuracy — at least with these four pellets in this one gun.
You may not remember this, but at the beginning of this series I said that if the results of three velocity and accuracy tests seemed to warrant it, I could do a fourth test with the velocities cranked down very low. I’ll hold off on the decision to do that until I see the results of the next accuracy test.
Whether I do a fourth velocity/accuracy test or not, I’ll write a final summary report on all that’s been done in this test. If it seems worthwhile, I could do a second test using a Talon SS, so we can see the same relationship in a PCP gun. However, that hasn’t been decided, yet. We still have to get through this one.