Twist, what is it, and how does it affect the job?
June 19th, 2019 By Allen Ford, GTI Tactical Instructor
Today's law enforcement is using the patrol rifle more and more. There are many factors for a Department or officer to consider when employing a new weapon system; here I would like to discuss one that is far to 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 bullets flight, and terminal being from impact to the final resting place of the bullet. The bore of a barrel is cut slightly smaller than the diameter of the bullet itself. Grooves are then cut into the bore leaving lands and grooves, much like threads. As the bullet is pushed down the bore by expanding gases it is forced to begin its rotation around the long axis. This rotation, or spin, is what stabilizes the projectile during flight. If it is not stabilized properly, depending to what degree, the external and/or terminal ballistics can be adversely affected. We will discuss the 5.56 x 45 in the AR platform here, as it is 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 weight, more compact cartridge to be used at close distances, 300 yards and in, for combat. The initial R&D was effective in combat because lethality of the bullet was considered. Part of this was a bullet that 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. As the military began to look at making the M-16 more accurate, particularly when it was being fired in cold dense air, as it sometimes would not be sufficiently stabilized for good accuracy. Also, once NATO adopted the 62 grain M855, which replaced the M193, a 1-14 twist rate was not sufficient. These factors lead to the military moving to a faster twist rate for the M-16. Faster twist rates lead to better stabilization of bullets causing them to be more accurate. However, most times it would enter and exit body tissue with little to no permanent or temporary wound cavity, also due to the fact that the bullets were better stabilized. The lethality of the 5.56 NATO cartridge is still an issue today. Let's keep in mind that lethality is affected by many factors including factors governed by the Geneva Convention, like bullet construction, not just the stabilization.
A heavier bullet needing a higher rate of twist in order to stabilize is a common misconception. Actually, a higher rate of twist is needed to stabilize longer bullets. Longer bullets of the same construction and the same diameter weigh more than their shorter counterparts, because the added length also adds weight. However, longer bullets need more spin because of the added length, not the added weight. If you had two bullets of the same diameter and same weight, but one was a lead core copper jacket bullet and one was a homogenous copper alloy bullet the homogenous bullet would be longer. Even though the bullets weighed the same, in theory, the optimal twist rate would be slightly higher for the longer bullet.
Every bullet fired has some pitch and yawl, torque is applied against the long axis, but this is decreased with proper stabilization. Even with the optimal twist rate it may take the bullet a certain amount of travel, generally around two hundred yards, in order to ‘go to sleep' or reach optimal stabilization. Think of a hand held object that is spinning, like a small fan. If you try to rotate the object against its rotational plane in your hand you may find it to be difficult, the faster the object spins. Turning the object with your hand would be similar to the pitch and yawl forces applied to a bullet in flight. Likewise, the greater the rotation speed of the bullet, up to a degree, the more stabile the bullet becomes. The stabilized bullet will be more accurate and have greater target penetration. Keep in mind that it is possible with some of the small thin jacketed bullets, such as varmint bullets, to be spun apart. This is a case in point as to how the twist rate can affect a bullets dynamics. In March of 1993, a barricaded suspect situation in Albuquerque New Mexico, ended with the suspect being shot twice by two separate officers both firing different rifles and different ammunition. The first officer was armed with a XM177E2 (Colt CAR15) with an 11.5 inch barrel and a 1-12 twist. The first officer was using M855 62 gr. (with penetrator) ammunition. The second officer was armed with a Colt HBAR with a 20 inch barrel and a 1-7 twist. The second officer was using Federal .223 69gr. Match (69gr. Sierra Match King) ammunition.
Both officers were positioned side by side approximately 15 yards from side 1. Both officers perceived the threat at the same time. Officer #1, with no intervening barriers, fires striking the suspect in the upper right chest / shoulder area. Officer #1's bullet then travels across the torso of the suspect and lodges in a kidney. Officer #2 fires at the suspect through an exterior wall of the barricade structure. Officer #2's bullet travels through 1 layer of T1 -11 siding, 4 inches of fiberglass insulation, and one layer of 1/2 inch sheet rock. Officer #2's bullet then strikes the suspect in the spine between the shoulder blades and travels through the chest and exits the sternum. The bullet then travels through another layer of 1/2 inch sheetrock, 4 inches of air space, and another layer of 1/2 inch sheetrock. The bullet then hits the freezer portion of a refrigerator and passes completely through. The bullet then hits a plate glass sliding door, passing through the plate glass, and was never recovered. In the aftermath of the shooting both weapons were tested and it was found that the M855 fired from the CAR15 with the 11.5 inch 1-12 twist barrel were unstable with bullets tumbling at 100 yards. The 69gr. Federal match fired from the HBAR with the 20 inch 1-7 twist barrel were accurate and stable.
Although the M855 design bullet was intended to enhance the penetration characteristics of the 5.56mm cartridge it hit the suspect's body, and was stopped in the torso without exiting. The 69 gr. Federal match was a “soft” bullet that was not designed to penetrate and it ended up penetrating a variety of barriers. The reason that there was such a difference in the penetration and stability characteristics of the two rounds is the compatibility and incompatibility of the bullets to the respective rates of twist used in the two carbines.
So, what is the best twist rate? There isn't one, but there are bullet and twist rate combinations that work and some that do not. A 1/9 twist rate is a pretty good all-around twist for an AR platform chambered in 5.56. But it seems as if sometimes the rifle and ammunition choices are completely arbitrary. Modern bullet technology has led to the development of bullets with very specific attributes. There are bullets designed for accuracy, to penetrate, not penetrate, and even disintegrate. Whatever the intended purpose of the bullet, it needs a proper platform in order to reach peak performance.