The AR-15 series of rifles is one of the most versatile long gun platform ever invented.
Developed in the late 1950s, the AR-15 is available in broad array of calibers, from .50 Beowulf to .22 Long Rifle. One of the most novel adaptations of the platform is to accommodate semi-automatic handgun calibers. The result is a weapon that appears similar to the basic AR-type weapon but which has significant differences “under the hood.”
The original AR-15 weapon was adopted by the U.S. military in the mid-1960s as the
M16 assault rifle. The M16 fired 5.56-millimeter ammunition and used the direct impingement gas operating system to cycle the weapon. Under the direct impingement system the M-16 siphoned off hot, high pressure gunpowder gasses from the barrel to cycle itself. The gasses drove the bolt carrier group (BCG) backward out of battery, ejecting the spent brass cartridge. As the BCG continued rearward it made contact with a spring-backed buffer tube which propelled it forward