So far in our discussion of external ballistics, we’ve covered bullet trajectory, elevation, the Coriolis effect, light effects, rifle cant, and bullet shape, but we still have one more important ballistics theme to explore: static stability.
The concept of static stability is essential to precision rifle shooting; if bullets are not properly stabilized, you’ll never get consistent results at long distances, and sometimes not even with short range shots. In the previous article about bullet shape, I talked about why a bullet needs to be stabilized through spinning. Now we’ll discuss how the spinning, or gyroscopic motion, works to keep the bullet stabilized during its flight. Also, we’ll learn how to predict how a certain twist rate is capable of stabilizing a particular bullet.
The spinning motion, induced by the rifling of the barrel, keeps the bullet stable because of gyroscopic effect. This effect limits, or cancels, the destabilizing momentum. It is the same effect that you can observe playing with a spinning top that prevents the toy from falling on its side—due to gravity—while it’s spinning. The higher the destabilizing momentum, the highet the gyroscopic forces must be to keep the bullet stable. The gyroscopic force is a function of the spin rate: the higher the spin, the higher the gyroscopic stabilizing force. Thus, bullets with longer destabilizing momentum levers need higher spin rates.
The spin rate of the bullet depends on the barrel twist rate, and on the muzzle velocity. The twist rate is the number of twist that the barrel rifling does in a given space. On rifles specification it is usually indicated the distance that a rifling take to make a full twist, in example 1:10 indicate that the rifling make a full twist every ten inches. Obviously, the shorter the twist rate, the higher is the spin rate. Given a twist rate, the higher is the muzzle velocity, the higher is the spin rate.