A Wee Spot Of Science

The practical physics governing aerial gunnery out of a moving aircraft is absolutely fascinating. Our M60 machineguns sported barrels rifled with a 1:12" clockwise twist. If my math is correct it means each M80 147-grain FMJ bullet travelling around 2,800 feet per second is spinning at 168,000 rpm. Any object freely subject to gravity accelerates toward the center of the earth at 32.17 feet per second squared. This is why when we shoot our weapons the bullets invariably arc downward until they eventually strike the earth. Factor in a 160-knot (184 mph) crosswind, however, and some of the most amazing things occur.

When firing bullets with a clockwise spin out the right side of a moving aircraft, the airflow across the spinning projectile creates a low-pressure area on the top of the bullet. Driven by the Magnus Effect, this causes each bullet to behave like its own miniature aircraft and fly upward. The practical result is a gunner trying to hit a target on the right side of a moving aircraft actually has to aim underneath it and let the bullets fly upward to impact where desired.

The same effect is manifest in the opposite direction on the left side of the aircraft. The low-pressure area is on the bottom of the bullet and causes the round to plunge precipitously toward the ground sooner than might be the case if fired from a stationary platform. The faster the aircraft the more pronounced the effect. In practice this makes a huge difference.