Pellet velocity versus accuracy test: Part 10
by B.B. Pelletier
Kevin Currie is shown shooting a tuned .177 Gamo CFX with his son and dog. He says his CFX is scary accurate!
For those who celebrate Christmas, Merry Christmas from Edith and me! This is our last opportunity to wish you a Merry Christmas before Sunday, but I would like to hear on Monday from anyone who received an airgun, airgun-related gift or a firearm for Christmas. I’ll tell you what I got, too.
Today, we’ll look at the results of this test to see if there’s a direct inverse relationship between pellet velocity and accuracy. I’ll start with the results by pellet and see where that goes.
A word about the shooting technique
The first accuracy test I did was in Part 2 of this report. I found fault with that test, though, because of how I was shooting. I wasn’t using the scope level on the gun all the time, and I also wasn’t “seasoning” the bore by shooting several shots before starting a group. Some pellets seemed to need the seasoning, while with others it didn’t seem to matter as much. I reshot the entire first accuracy test and seasoned the bore for every pellet, plus I paid attention to the scope level.
The need for seasoning seemed to go away as testing progressed, but the scope level was always consulted for every shot. I know that the level improved the performance of every pellet that was shot. The jury is still out on the seasoning issue.
All the accuracy results seen here are not from the first time I shot the rifle, but the second. All were shot at the velocities indicated. Just the shooting techniques were adjusted as indicated.
In this test, the 7.1-grain Beeman Devastator was the “little pellet that could.” From the start, when it was averaging 1,216 f.p.s., this lightweight hunting pellet produced 10-shot groups under three-quarters of an inch at 25 yards. That went against the popular belief that supersonic velocities are harmful to accuracy.
The Devastator turned in the following performance at 25 yards.
Velocity (f.p.s.)….Group size
Okay, you don’t need a graph to see a problem here! This pellet is obviously way more accurate at 1,123 f.p.s. than it is at 772 f.p.s. Theory says that shouldn’t be because the first velocity is breaking the sound barrier, which is where all the accuracy gremlins are supposed to live.
Looking at the group size in relation to the velocity, it appears that 1,123 f.p.s. is the most accurate velocity for this pellet in this gun. That would entirely negate the theory that velocity destroys accuracy. So, if there is such a relationship, it must be subordinate to and less influential than some other influence. I think that other influence might be vibration, but that’s just a guess.
Crosman Premier lites
The 7.9-grain Crosman Premier lite was the next pellet I tested. Here are the results of all four tests at 25 yards.
Velocity (f.p.s.)…Group size
The Premiers did give a linear relationship between velocity and group size, though the two groups from the middle two velocities are so close in size that they could be the same. Measuring error is greater than the difference between these two groups.
The group at the lowest velocity is obviously the best of the four and by a wide margin. The data from this pellet isn’t clear as to what is causing the accuracy improvement. It could be either velocity or vibration. However, at 915 f.p.s., the pellet is going slow enough to be out of the transonic region, while at 1,057 f.p.s. it isn’t. I would have expected to see an accuracy gain at that lower velocity that’s greater than what we see here if the real problem is just velocity.
Next, I shot the heavyweight Beeman Kodiak pellet. It proved to be the most accurate pellet of this test and the only one that turned in a group smaller than a half inch. Please bear in mind that these are all 10-shot groups and are about 60 percent larger than they would be if they were only five shots. I didn’t shoot 10 shots for that reason — but because, in doing so, I reduced the probability error significantly. In short, I can trust that the group sizes shown are closer to reality that if I had shot two 5-shot groups and averaged them.
Kodiaks shot tight right from the start, even though the first group of pellets was actually close to or just within the transonic range. Since I didn’t keep atmospheric data for each day I shot, I can’t say where the sound barrier was exactly; and the transonic region, which is 0.8 mach to 1.5 mach, is calculated based on that. In practical terms, I know that 992 f.p.s. is pretty darn close to transonic, because the sound barrier can be anywhere from 1,050 f.p.s. to 1,125 f.p.s. depending on the temperature and humidity where I shoot.
Velocity (f.p.s.)…Group size
However, the groups don’t seem to support the velocity/accuracy relationship very well. It’s true that the first and second groups are too close to really say which is larger than the other, but the velocities at which they were shot are very important. One borders on the transonic, while the other is probably slower than transonic.
But look at that third group! When the velocity averages 819 f.p.s., the Kodiak loves this rifle! Is that a velocity thing or is that influenced by the harmonics of the rifle at that power level? I’m inclined to think that it’s the latter, though we do not have enough data to prove it.
The last group is the worst, though the velocity is getting pretty low for a pellet this long and heavy. We don’t know much from these results, either. But if it does turn out to be harmonics over velocity, then this pellet is probably the best one for the rifle, and the Harmonic Optimized Tuning System (HOTS) needs to be adjusted for it at around 900 f.p.s.
The 16.1-grain Eun Jin pellet is too heavy for the power potential of this air rifle. Although the Whiscombe JW 75 is a 30 foot-pound air rifle — that’s only in .25 caliber — when the heaviest pellets are used. In .177, it’s much closer to 20 foot-pounds and is, therefore, too weak to drive the 16.1-grain .177 Eun Jin fast enough for accuracy.
Velocity (f.p.s.)…Group size
Starting with the second group, I could tell that the Eun Jins weren’t going to do well. Like the Kodiaks, they’re also too long and heavy to make any conclusions regarding velocity versus harmonics. They just don’t give enough speed in this rifle to properly examine the velocity question.
A different look at the data
The “sweet spot” was different for every pellet. Each pellet had one velocity at which it grouped the best; disregarding the actual velocity, it went like this:
Fastest was best—>Eun Jin
Second fastest was best—>Devastator
Third fastest was best—>Kodiak
Slowest was best—>Premier lite
That, by itself, is a pretty good indicator that supersonic speed isn’t a problem, since the Devastator was supersonic when it produced its best group. One thing you cannot do is compare the group sizes…one to another…between the pellets. Let each pellet stand alone because there are far too many variables to make a cross comparison like that.
Was the Whiscombe a valid testbed?
Some thought the Whiscombe was the wrong gun to use because it’s so inherently accurate. I disagree. I think its accuracy makes the results all the more valid. Besides, controlling many of the magnum breakbarrel springers is too difficult and gets in the way of testing. They require perfect hold technique for every shot. The Whiscombe is much easier to control, which takes that variable out of the equation.
Some felt that only a pneumatic should be used since harmonics seemed to be causing accuracy errors. Well…that was the point of testing! Now we know a little more about how the pellets respond to supersonic speed, and it seems to me that it doesn’t matter as much as many, including me, have believed. People don’t just shoot PCPs. I do plan on testing this same sort of thing with a PCP whose velocity I can control over a wide range, and those results will also be interesting — but they don’t negate the value of testing a springer.
I used the Whiscombe for this test because, even when I altered the velocity, the harmonics of the gun remained the same. The powerplant always ran at full power, regardless of how fast it shot. And the barrel was always the same, too. The only thing that changed was the velocity.
What comes next?
I’m sure some of you will have additional interpretations to make about this test, and now is the time to make them known. My next step is to use the rifle as it is currently set up and adjust the HOTS to see what I can do to the size of the Beeman Devastator group. I’m thinking the HOTS can be adjusted to shrink it significantly. I have a procedure in mind to cut the time it takes to adjust the rifle because this can take hours if you aren’t careful! I’ll share that procedure with you in that report.
If I’m right about being able to tune the HOTS to get a small group at the average of 772 f.p.s., then the next thing I’ll do is shoot another group with the bubble level taped to hide the bubble. I won’t intentionally try to enlarge the group, but I just won’t be able to consult the bubble for every shot. That will result in another group that can be compared with the best group I’m able to shoot when the HOTS is tuned, because it will still be tuned for that group.
Finally, perhaps one additional test is needed. I’ll set the rifle to shoot Kodiaks in the high 800 f.p.s. range and adjust the HOTS for the best accuracy. When I get the best group, I’ll shoot one group of weight-sorted pellets against another group of pellets selected straight from the tin.
When all of this is completed, I should be able to state what I think are the most important components of accuracy. You’ll be able to see how much difference these things really have on group size. Most of you can’t adjust the harmonics of your spring guns, so you need to find the one pellet that shoots the best. Perhaps it’s time someone made a generic harmonic tuner for springers, again.