.22 bullet flight

[Rifleman]

Gentlemen (and ladies):
I come to you from the world of International rifle shooting. I have become involved in learning why a .22 bullet, traveling at less than 1100 fps, does what it does in differing air densities. Specifically, we are attempting to decipher why flight stability (and hence group size) appears to improve when using "faster" ammo as the air density increases in response to altitude and/or barometric pressure, but slower ammo seems to improve groups when humidity goes up.

In other words, we think we have observed that the same ammo speed gives the best groups in a high or low DA independant of humidity; but once humidity rises, we see group size shrink by changing to slower ammo.

Please understand, I lied when I registered. -- Not an engineer of any type! But I am in the middle of this controversy and we need to know if there's an answer.

Is there something about humid air that changes flight stability in a decelerating projectile?

thank you!

Rifleman

 

[btrueblood]

Oh, and somewhere somebody got it backwards - humid air is less dense (lower molecular weight) than dry air at the same pressure and temperature. Humid air thus has a higher speed of sound, i.e. a bullet with a given muzzle velocity will be flying at a lower mach number in humid air than in dry air.

The trouble with moisture in the air is not a molecular effect, but has to do with condensation and the formation of droplets. A water droplet of even a few microns diameter is significantly heavier than an air molecule, and the impact with a solid body no longer follows the "laws" of gas dynamics. Lots of random impacts with tiny (but not nano-scale) droplets during the bullet time of flight could increase the bullet CEP. Condensation phenomena are generally ignored in most airframe aero analyses - the drops are tiny relative to the weight of a fighter jet. But a .22 cal bullet is getting pretty tiny relative to a fighter jet too. Condensation is considered when doing subscale supersonic wind tunnel testing - high velocity droplets have a way of sandblasting the nice, polished surfaces of tunnel models. My $.02.

 

[aviat]

You might be interested with this:

http://www.riflebarrels.com/articles/bullets_ballastics/density_bullet_performance.htm