airgun velocity: design physics

It seems there are still those that are determined to explore fireing conventional airgun projectiles at high speeds with little real thought on whats possible let alone what is practicable in terms of stability, accuracy and energy retention.

Theory is fine when moderated with an understanding of basic physics. Let me explain: Whilst in theory my 11ft/lb energy air rifle will project my 1.2gr Prometheus ZAP to 2031.55fps its highly unlikely to be a viable projectile!

The limiting factor on the speed of an airgun or even powder burning firearm is the speed of sound in the working fluid (in this case air). Essentially this is because the projectile is accelerated by the pressure difference between its ends and such a pressure wave cannot propogate any faster than the speed of sound, in the medium used.

The speed of sound in less dense gases increases dramatically with helium being 3 times that of air and hydrogen as used in light gas guns some 3.8 times that of air. The speed of sound also increses in relation to an increase in the temperature of the medium ( but remains independant of pressure), so the heat formed by the compression of the working fluid serves to increase the maximum possible speed.

Spring piston airguns, heat the air enough to combust some of the piston lubricant (dieseling): this raises the speed of sound in the compressed air enough to overcome frictional and other efficiency losses and thus propel the projectile at more than the speed of sound in the ambient conditions.

Now that we understand the basic precepts of design physics we need to look at exactly what influences come into effect when we do push airgun projectiles (or any projectile) at near sonic velocity. It is then that we will begin to understand why seeking excess speed becomes an excercise in futility !