Central to the M4 problem is the question of lethality. Is the M4 sufficiently lethal to do the job, and to remain the standard issue carbine for the U.S. Army? To answer it we need to dimension what constitutes lethal; regrettably easier said than done. Anything I’ve been able to find lacks sufficient specificity; thus leaving the concept of lethality a moving target. This gives me latitude, as a writer, to define it as “the ability to completely incapacitate a target with a center of mass hit out to 300 meters.” I’m sure the Program Manager for Munitions has a tighter definition but I’m in all probability not far from the Army’s concept of lethal. I also want to discuss the mechanics of lethality.
There is considerable amount of forensic data to help us; much of which is very technical. However, it can generally be said that for a projectile to be lethal, it must penetrate the target sufficiently to cause massive tissue trauma. It must destroy vital organs and create a large wound cavity so that blood pressure drops instantly incapacitating the target. There are empirical measures that establish the low end of lethal at approximately 60 foot-pounds of energy; others say 120 foot-pounds. Somewhere in that vicinity lies the correct number.
Forensic reports generally consider kinetic energy to be the least influential external ballistic property to affect lethality; the cross sectional area of the projectile is generally considered to be the most influential. So, it follows that larger calibers will have a greater lethality than a smaller one. Let’s revisit the question – Is the M4 sufficiently lethal to do the job?
Today, the M4 / M16 family of weapons are chambered to use a 5.56 mm x 45 NATO round. The original projectile was a 55gr full metal-jacketed bullet, M193 Ball, which when fired from the 20-inch barrel of the M16 exits the muzzle at over 3000 feet per second. In contrast, the M4 with a 14.5 inch barrel has a muzzle velocity of approximately 2800 feet per second, a difference of 200 feet per second. As it turns out, the 200 feet per second difference is significant, and this is why.
Kinetic energy is the force that the projectile will impart to the target on impact. Its formula is (mass * v2) /2, where mass is the weight of the bullet and v is its velocity. Increasing the mass of the projectile increases kinetic energy and decreasing it, decreases it. On the other hand, velocity is a squared function so a change in velocity will have a greater effect on kinetic energy than a change in mass. So, by standardizing on a carbine with a barrel length of 14.5 inches v. 20 inches, of the M16, we also reduced muzzle velocity by 200 feet per second, and correspondingly reduced the projectile’s kinetic energy much sooner along its flight path. The reduction in kinetic energy means the bullet will not tumble and fragment properly on impact. There are other complications that creep into this lethality question.
Soldiers don’t generally engage in drive-by shootings; instead, they assault positions where the opposing force is typically entrenched with barricades and barriers in place. The assaulting force attempts to pin these forces down then move against the objective. It follows that a soldier needs to be able to shoot through a barrier to hit his opponent hiding behind it.
In the 70’s, SS109 or its U.S. designation M855; commonly called green-tip was developed. The M855 round uses a steel core jacketed bullet designed to penetrate a steel helmet at 600 meters. It also uses a bullet weight of 62 grains rather than the 55-grain bullet found in M193. The changes incorporated in the M855 produced improved penetration and greater stability at longer ranges but it comes at the expense of a reduced muzzle velocity. So, what happens when we shoot a soft target using M855 – the projectile passes through the target with the bullet still intact; simply leaving a .223 diameter hole in the target. Because the bullet does not fragment or tumble on impact, its lethality is adversely affected; requiring the soldier place multiple shots on the target. Hence the outcry “…we’re hitting them but they are not dropping…we have to hit them two and three times…” Therefore, is the M4 sufficiently lethal is not a very good question. The better question is, should the 5.56 x 45 remain the caliber of choice? Currently there is a lot of work being done on the ammunition side and designers are focusing on bullet geometry and composition including hydrostatic properties.
Regardless of the platform ultimately selected, lethality will not improve without a meaningful redesign of the 5.56 x 45 NATO round, or the transition to a larger caliber.
I’ve included the results of a NATO study that points to marksmanship and marksmanship training. I’m not entirely in agreement with the panel, but they raise interesting and valid concerns.
 Bullet weight has other dynamics such as ballistic coefficient, stability at long ranges and rifling to name a few.