The M16 is the United States military designation for
the AR-15 rifle adapted for semi-automatic, three-round burst and full-automatic fire.
AR-15 is a lightweight, 5.56 mm, air-cooled, gas-operated, magazine-fed
semi-automatic rifle, with a rotating-lock bolt, actuated by direct impingement gas operation. It is manufactured
with the extensive use of aluminum alloys and synthetic materials. The name "AR-15" is a Colt registered trademark,
which refers only to the semi-automatic rifle.
The M4 carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16 rifle,
all based on the original AR-15 rifle. The M4 is a shorter and lighter variant of the M16A2 assault rifle,
with 80% parts commonality.