RIFLE BALLISTICS by
What travels at over 540 miles per hour and spins at a theoretical 24,000 revolutioms per minute? It might surprise you to know that it is a .177 pellet fired from an air rifle.
Most rifles have rifling with half to three quarters of a turn per foot. Therefore, with a muzzle velocity of 800 feet per second and a pellet weight of 8.4 grains, which gives 11.94 foot pounds, and with rifling of half a turn per foot, (assuming that there is no stripping in the rifling), the pellet would leave the barrel at over 24,000 revolutions per minute. This brings into question the uniformity of density around the circumference. As you all know, the out of balance of road wheels of a car causes steering problems, (a typical wheel turns at 1000 revs per minute at 70 mph). so just imaging the result if a pellet is slightly out of balance. Another problem is the centrifugal tension acting on the periphery. In other words, trying to tear it apart. In our case this amounts to 1.22 pounds. A thin skirt could splay out causing the pellet to be deflected from the chosen path by way of the unequal aerodynamic pressure put upon the damaged skirt.
It all adds up to more excuses as to why you miss the
target. But does it? The idea of the spin imparted to the pellet is to try and minimise the effect of slight
imperfections. The pellet will travel in a spiral towards the target and only gross damage to the skirt will have a pronounced
effect. This was well illustrated in early musket balls where the ball had to be hammered into the muzzle before ramming home with the ram rod.
Another reason for deflection is the wind strength and direction. Most air rifles have the rifling twisting clockwise looking from the breech end. If the wind is coming from the left the impact point will be high right, whereas it will be low left if the wind is from the right. Air density, humidity or dust particles also have an effect.
One thing not always fully understood is that the pellet velocity and rifling are closely matched. The selection of suitable pellets for the rifle are well known. An increase in velocity might give more power but it does not always follow that the accuracy will not be impaired. Increased power does not give increased accuracy, consistency shot to shot does. The increase of velocity can cause stripping of the pellet in the rifling and a poor choice of pellet can effect accuracy, which is not always noticed at short range. Another reason not to increase velocity too much, in a PCP rifle is the reduced number of shots to the fill.