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What destroys airgun accuracy? Velocity or harmonics?

I've been an active airgun writer for the past 18 years, and I've been a shooter for more than half a century. In all that time, I believed that keeping velocity under the transonic region was crucial for the best accuracy.

I could even cite reasons for believing the way I did, and the majority of airgunners who investigated the subject believed as I did. I warned people not to let their airguns go over 950 fps, lest they suffer the fate of dwindling accuracy. I even showed them shot groups to prove my point.


Now, I doubt whether any of what I thought to be true actually was. I recently conducted a lengthy test in which four different pellets were tested at four different velocities--all in the same airgun. I have a spring-piston rifle made by John Whiscombe that allows me to vary the velocity without changing anything on the gun other than the air transfer port.

The same barrel was used with the same powerplant to test four different pellets--each shot at four different average velocities in a test I thought would prove once and for all that pellets have to go slower than the transonic range to be accurate.

The only problem was that my test proved exactly the opposite. The fastest pellet was considerably more accurate than the same pellet going much slower. This raises the question of whether it's actually the pellet's speed that causes inaccuracy or the harmonics of the rifle that accompany that speed. Because my Whiscombe rifle allows me to tune the harmonics of the barrel to the optimum point, I can test that question.

What's Bad About The Transonic Region?

The transonic region exists from Mach 0.8 (893 fps) to about Mach 1.1 (1228 fps). Modern pellet guns shoot in this velocity range much of the time especially in.177 cal.

Mach 1 is the speed of sound, which varies with the air tempera-ture, barometric pressure and humidity. I live just above sea level, and Mach I can vary from just over 1050 fps on some days to as much as 1115 fps, depending on all the variables. If you live close to the same altitude and shoot .22 rimfire subsonic ammo, you'll notice that on some days the rounds don't break the sound barrier and on other days they do. And on some days, one round will be subsonic while the next will crack as it breaks the sound barrier--it's that close.

The Theory

The transonic region is just below the actual sound barrier; and the popular theory is that when a pellet gets going that fast, it creates a bow wave of air pressure that can flow around the pellet and hit the tail, causing the pellet to flutter in flight. Or, if the pellet is flat-out supersonic at the muzzle, which isn't uncommon these days, it will soon slow down to the transonic region--where all these bad things start happening. Either way, a pellet that goes fast will suffer this disturbance and be less accurate, or so the theory goes.

My test was constructed to demonstrate, in an irrefutable way, just how damaging this effect can be to accuracy. I shot 10-shot groups I instead of five-shot groups because I wanted the bad groups to stand out dramatically from the good ones.


And stand out they did, only things didn't go the way I thought they would! I started the test with the rifle set to shoot a lightweight Beeman Devastator hollow-point pellet at more than 1200 fps. I shot a 10-shot group at 25 yards and used a scope level before every shot. According to my beliefs, the pellets should have made a group of over 1 inch, center-to-center, and more likely around 1.5 inches.

Imagine my surprise when the pellets that averaged 1216 fps made a 10-shot group that measured only .743" between centers!

When I adjusted the rifle to shoot the same pellet at an average of 1123 fps, which is still over the sound barrier, it made an even smaller .616" 10-shot group. Yet, when I slowed this same pellet down to an average of 772 fps, a speed I would have bet was going to be the most accurate, the group opened to 1.073 inches, again for 10 shots at 25 yards!

Those results flew in the face of everything I'd believed and had written for almost the past two decades. But I wasn't finished testing the theory because this rifle was also capable of having its harmonics adjusted.


Adjusting the HOTS

The Whiscombe has an adjustable barrel weight at the muzzle that can be moved in very small increments to balance the vibration nodes for a given pellet, theoretically giving you the ability to adjust for the best or tightest group possible with that pellet at any given speed. Whiscombe calls his system the Harmonic Optimized Tuning System. or HOTS. It's not much different than the Browning B.O.S.S. found on their centerfire rifles, and they have a lot of powerful advertising claims that say their system does work.

My next endeavor was to adjust the HOTS with the rifle shooting at an average 772 fps--the velocity at which it was grouping more than one inch for 10 shots at 25 yards--and see if the group size could be made smaller. If it could, then harmonics and not velocity would be the only variable that changed.

I won't walk you through all that I did to adjust the HOTS for this one pellet at that one velocity, but suffice to say it took me about four hours over two different clays to make all the adjustments and test each of them with 10-shot groups. I was shooting with a scope level for every shot, so this testing was both long and tedious.

But it paid off in the end, because I finally arrived at a HOTS adjustment that produced a 10-shot group measuring .523" between centers--or just larger than half the size I started with. Clearly, harmonics and not velocity were affecting the size of the groups.

Two Important Points

Two things stand out in this test. First--the pellets made tighter groups when they went faster than the speed of sound than when they went considerably slower. Now that I have all the information, I know that this was a lucky accident where the HOTS happened to be adjusted when I started the test.

Second--it was possible to tune the harmonics of the gun to tight-en the group when the gun was shooting slower, so it printed a group just over half the size it had made before the harmonics were adjusted. The slower velocity was not the cause of less accuracy by itself--it simply existed at a spot where the HOTS was out of tune with the vibration nodes at that velocity.


So What?

You don't own a Whiscombe air rifle and your rifles don't have the HOTS on them. What possible good can you get from this test?

Here's what this means to you. Say you have a spring rifle that prints a group of 10 with a tight cluster of six, surrounded by four "flyers." Only you know they aren't really flyers, because you watched each shot. Nothing distracted you. These were perfect shots, yet four of them decided to move away from what would have been a very nice group.

In the past, if you had reported this problem, we would have asked you pointed questions about the velocity of the pellet you shot in the gun and if it was too fast, which we would have said about any pellet going above 950 fps at the muzzle. We would have advised you either to shoot heavier pellets in your gun or get your gun tuned to shoot slower.

The strange thing is that this advice might have fixed your problem, because it would have changed the harmonics of the gun. A heavier pellet would have exited the muzzle later in the shot cycle, perhaps aligning with a spot in the barrel vibration sine wave where the muzzle was always in the same place, or it would have changed the vibration patterns of the gun (changed the shape of the sine wave, itself) and pretty much done the same thing.

Had we advised you of this "cure" and had you applied it and found that it fixed your problem, we would have been like witch doctors who buried a toad at the full moon and if the problem went away, we would have put toads on our list of prescribed cures.

With the data from this test, and there's much more information than what has been shared here, I now think that harmonics and not velocity is the largest contributor to accuracy problems.

But the solution for most shooters who have airguns that shoot at fixed velocities and don't have the ability to adjust the harmonics are still the same. Find a way to slow down or speed up your pellet or tune your gun to change the vibration patterns. If you're very clever, find a way to change the vibration harmonics of the gun.
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Author:Gaylord, Tom
Publication:Shotgun News
Date:Apr 20, 2012
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