As the video notes, 55-grain M193 FMJ/Ball ammunition was intended to be used from an AR15/M16 with a 20-inch barrel. The U.S. Army’s “light rifle” competition was instigated by U.S. Army General Willard Wyman, the commanding general of CONARC, in the mid-1950s. Since the specification set forth by Wyman’s command called for a .224-caliber bullet and also wounding potential equivalent or superior to the M-1 Carbine, that’s what Eugene Stoner and his team used as their starting point. Stoner later explained that in order to get a projectile as light as 55-grains to do the job, he exploited the known dynamic instability of lightweight, high-velocity projectiles moving at high speeds. Specifically, their propensity to yaw in flight and then “swap ends” (tumble) when encountering a target. As an aside, it is important to note that this propensity has been misinterpreted over the years to mean that the projectile (bullet) itself tumbles while in flight. This is incorrect. The whole point of rifling a barrel is to spin-stabilize the projectile by spinning it at very high rpm. Rather, the yawing and subsequent tumbling occur when the bullet encounters a target such as living tissue. Usually, “keyholing” on a paper target is evidence of dynamic instability and an insufficiently rapid rate of twist to stabilize the projectile. Stoner’s team increased the wounding potential of the 55-grain bullet by including a cannelure or crimping groove in the design, at which the bullets would often fracture or shatter if moving at sufficiently high MV. Instead of a caliber-sized hole, multiple high-velocity wound tracks would be created, thereby increasing the potential lethality of the projectile. A modern pointed (spitzer) rifle bullet is tail-heavy anyway, since the center of gravity is usually aft of the midpoint of the projectile. It is kept in its proper nose-first orientation by the high rate of spin imparted by the rifling and the velocity of the shot itself. This orientation breaks down only when the bullet has traveled far-enough to shed much of its velocity and rate of spin, or when the projectile encounters a target of some kind which slows it down dramatically. It is then that the projectile will want to “swap ends” into a tail-first orientation, possibly shattering if velocity is still high-enough. After-action reports coming out of Vietnam painted an inconsistent picture initially. Some troops reported stunningly-effective terminal performance and combat effectiveness, whereas others reported indifferent terminal performance and a failure of M193 to put down enemy “tangos” hit center-mass. It is now thought that these reports merely reflected the somewhat idiosyncratic characteristics of the M193 round, its dependence upon certain conditions for optimal effectiveness in combat. These findings were later confirmed by rigorously-controlled studies done at Crane Naval Special Warfare Center and elsewhere, but at the time, they sparked an intense controversy within military circles about the relative worth of the M-16 and its 5.56x45mm ammunition which has still not abated today. The new 62-grain “green-tip” M855, the NATO standard going forward from the 1980s,… Read more »