The US service rifle in use since 1965. The M16's original design proved to be jam prone (complicated by poor ammunition and improper cleaning instruction) and was superceded by the M16A1. The M16A1 was a fully automatic rifle with a chrome lined barrel for increased reliability.
The M16A2 was the next major model which had an improved sight, the optional addition of a brass deflector for left-handed shooters a 7-in-1 rifiling, and, most notibly, a three-round burst limiter.
The next model of the M16 (the M16A3 or M16A4 I cannot find reliable sources on either even Colt) eschews the burst limiter and adds a P-rail inplace of the handgrip.