Today’s blog comes from the November 1995 issue of The Airgun Letter.
A conversation with another airgunner triggered the start of an investigation into pellet ballistic characteristics. He said he was getting some shifting groups, left to right, at different ranges. His rifle is a TX200 shooting 7.9-grain Crosman Premiers. As you will see, velocity isn’t an issue (although it could be a cause).
His question prompted me to examine some targets I shot with a Daystate Huntsman. The groups I got using 7.9-grain Premiers also shifted from left to right at different ranges. Additionally, the height of the groups centers differs from the expected trajectory for that pellet at the velocity I was shooting.
I can think of two reasons for what seems to be happening. Either the wind is moving the groups or else the pellet is traveling down range in a spiral path. I ruled out the wind because of the tightness of the groups and because the wind was under 3 mph on the day they were shot. That leaves spiraling as the most likely culprit–assuming I am right in my suspicions. For the sake of discussion, let’s say I’m right and the pellets are spiraling.
The only thing I can think of that would cause spiraling is an unstable (yawing) pellet that precesses around its axis in the direction of the spin. If you have ever seen a washing machine become unbalanced on the the spin cycle and hop around the floor in a certain pattern, you have witnessed the phenomenon of precession.
That bullets can precess has been known for over a century. I believe it was discovered very shortly after elongated bullets were first used in rifled barrels, years ago, I read an article in The American Rifleman about a test on the brush-bucking ability of a .30 cal. bullet. Once stability was disturbed, the bullet began to precess in the direction of twist in an ever-increasing spiral. Of course, that test is not the same thing I’m discussing here, as the instability there was induced mechanically down range by the bullet striking a broomstick rather than yaw at the muzzle. But it does show that bullets can travel in a spiral path.
Bullets (and pellets) can be made unstable by their twist. Varmint shooters are aware that thin-jacketed bullets have been known to explode in flight from the centrifugal force of their spin. And tumbling, or more probably precession coupled with pronounced yawing, is well-known from the early days of the M-16′s development. I remember that a rifleman had little chance of hitting a man-sized target at 300 yards with early M16 rifles. The bullet design/twist rate combination had not been worked out correctly during that time.
With a right-hand twist, the precession spiral would be clockwise. I would also expect the spiral to enlarge as the pellet gets further from the muzzle.
I’m not too concerned about what causes this thing, if it’s happening. As a shooter, I’m more concerned about not using a pellet that does it. The causes for instability/precession are too numerous to discuss in just one blog. I have, however, drawn a picture of one thing that could cause precession to start. I don’t say it’s happening this way, but it might. The pellet could be yawing from instability as it leaves the barrel, causing an area of low pressure to form behind the skirt, where it sticks out in the the air stream. This low pressure causes uneven drag, which starts the pellet precessing about its axis. None of this is new, of course; it has been discussed in the literature for over a century.
What may be new, or at least a rethinking of an old problem, is the ease with which an airgun pellet might be induced to precess. The drag on an airfoil increases with the square of the velocity, I believe. That’s why the “sound barrier” presents such a problem to powered flight. Unless the flight surfaces are correct for transonic airflow, the increase in drag can literally tear apart an airplane.
Of course, bullets and pellets don’t tear as easily as airplanes, but they are affected by the increase in a drag. If a pellet were tilted on its axis, relative to the direction of travel, the drag on it would have to be uneven and the amount of uneven drag would grow quickly as velocity increases.
Okay–so what does all this toffee-nosed drivel mean to real airgunners? It means that even if you correctly adjust your scope for trajectory, there’s still a big chance you won’t hit that half-inch kill-zone at 15 yards. Not because you’re too high or too low, but because you are too left or too right! If you’re throwing a spiral and your pellet isn’t centered on the line of sight at the range you expect it to be, you could miss.
All I’m doing here is reporting on a phenomenon, which may or may not be happening. I don’t know if it is. I’m speculating, based on a small observation. Others I’ve talked to have either seen this same thing, or they think they may have read something about it somewhere. Do you think some pellets spiral?