By GTI Instructor Allen Ford
Today's law enforcement is using the patrol rifle more and more. Departments or officers must consider many factors before employing a new weapon system; here I would like to discuss one factor that is far too often over looked or misunderstood. Twist, what is it, and how does it affect the job?
In the law enforcement or military community, it is important to have a basic understanding of internal, external, and terminal ballistics. Internal; being from primer strike to muzzle exit, external; being the bullet's flight, and terminal; being from impact to the final resting place of the bullet. The bore of a rifle barrel is cut ever so slightly smaller than the diameter of the bullet itself. Grooves are then cut into the bore leaving lands and grooves, much like threads on a screw. As the bullet is pushed down the bore by expanding gases it is forced to begin rotating around its long axis. This rotation, or spin, is what stabilizes a projectile during flight. The rate of this rotation or spin is called the twist rate. The stabilization or lack of stabilization, created by the twist rate can affect external and/or terminal ballistics. For example, higher stabilization results in better accuracy and penetration, where lower stabilization results in earlier tumbling.
We will discuss the 5.56 x 45 in the AR platform here, as it's the most prominent cartridge and weapon platform employed today by law enforcement and military. The research for the original M-16 began with the military looking for a lighter, more compact cartridge to be used at close distances, 300 yards and in, for combat. The initial research on the M-16 showed that it was effective in combat because the bullet had a high lethality. This high lethality was partly due to the fact that the bullet was marginally stabilized. The one-in-fourteen twist rate would rotate the bullet just enough to stabilize the original M193 55 grain projectiles. When the bullet impacted, it was prone to tumbling, giving it good lethality. Often times, as it began to tumble, the force exerted across the side of the bullet would tear it apart and cause tremendous fragmentation.
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