As the AR-15-type rifle approaches the half-century mark since its adoption by the US military, it is difficult to comprehend that it represents the longest serving rifle in American military history, exceeding even the classic M1903 that served in one form or another from 1903 until the late 1940s. The tactical AR has taken two basic directions with the US military since its adoption in the mid-1960s. The Army has settled on the short-barreled M4 carbine as its weapon of choice, while the US Marine Corps has adopted the full-length M16A4 as its standard rifle, and for good reason. For law enforcement, the reasons behind the Marines’ continued use of the full-length rifle and a comparison between it and the M4 bear examination before we get into our test rifle.
M4 versus M16
It is no great secret that the M4 carbine suffers from lethality issues in Iraq and Afghanistan. “Bad guys” shot at distances greater than 50 to 75 yards have the disconcerting habit of not being disabled and continuing to fight. This was noted as long ago as 1992 in Somalia and was actually reported in the bestselling book, Blackhawk Down. The issue is the velocity at which the bullet leaves the weapon’s muzzle. The 5.56mm NATO round was designed to be fired from a rifle with a 20-inch barrel, giving a muzzle velocity of some 3000 feet per second (fps). The bullet is stabilized in flight by the spin imparted by the rifling. As velocity and spin slow, the bullet wants to return to its naturally stable state, base forward. With a rifle, this occurs at approximately 200 yards when the bullet’s velocity drops below 2400 fps. If a 5.56mm bullet strikes flesh at a velocity greater than 2400 fps, it slows quickly and begins to turn base forward. At about halfway through the turn, the bullet blows apart into fragments, causing a large permanent wound cavity, disabling the target.