Heckler & Koch HK416 is a rifle largely based on what variant of the M16A2?
Heckler &amp; Koch HK416:  Although its design is in large part based on the AR-15 class of weapons, specifically the Colt M4 carbine family issued to the U.S. military, it uses an HK-proprietary short-stroke gas piston system originally derived from the ArmaLite AR-18 (the same system was also used in Heckler & Koch's earlier G36 family of rifles). M4 carbine: The M4 carbine is a shorter and lighter variant of the M16A2 assault rifle.
M4 carbine