THE OFTEN UNDERRATED AIR GUN
(Page 2 of 6)
May/June 1981
Excerpt by Jim Carmichel
When the trigger is pulled, the powerful mainspring slams the piston through the chamber, driving and compressing the air in front of it. This body of air escapes through the barrel, driving the projectile.
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The relative projectile velocity of a spring-piston air gun is determined by a number of factors, but velocity is mainly a function of the speed and power of the piston and the volume of air displaced. Other factors are the weight and size of the pellet, and the amount of air leakage which occurs around the projectile and piston. Naturally, a projectile which efficiently seals the bore, and a close (but not tight) piston-to-chamber fit are more efficient than tolerances that permit air blowby.
For illustration, look inside a more or less typical Daisy BB gun. When the gun is cocked, the piston is levered back against the spring, which in turn is "loaded" to a force equal to about 60 pounds. When the trigger is pulled, the spring drives the piston forward in the chamber for a distance of slightly over 2 inches.
In everyday ballistic terms, the piston velocity tops out at about 41 feet per second. Why then, you may ask, does the BB projectile have a muzzle velocity of over 300 fps? First of all, the piston and spring are not pushing the BB itself, but rather a column of air. Since this moving column of air can escape only through the small hole in the barrel, it obeys the laws of gas dynamics and speeds up in order to move its volume out of the chamber. This means that the air in the barrel moves faster than the air immediately ahead of the piston. This is what gives the BB its first big push. The second kick is supplied by the air compressed in the chamber.
At first the BB doesn't want to move (because of its inertia), causing the air to "pile up" behind it and become compressed. In fact, the piston has traveled a half-inch before the BB moves at all. This means that the air in the chamber is being compressed, eventually reaching a pressure greater than 300 pounds per square inch! Even after the piston stops, this compressed air continues driving and accelerating the BB. At the moment the piston stops its forward motion, the BB has a velocity of some 175 fps and has traveled less than 2 inches up the barrel. At this point the compressed air, which is near its peak, takes over and accelerates the BB to about 300 fps during its next 3 inches of travel. As the BB moves up the barrel, it reaches a velocity of about 330 fps.
Naturally, different makes and models of spring-piston guns generate different pressures and velocities.
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