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instruction) and was superceded by the M16A1.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.