The M16, M16A1 and M16A2 are military assault rifles with full
automatic or burst-fire capability.
The civilian version is the AR15 which lacks the full auto or burst
feature. It is capable of only semi-auto fire (i.e., one round
discharged per trigger pull).
Externally, both versions are identical (except for an additional
position on the selector switch of the M16 that is labeled BURST or
AUTO); internally, the mechanisms are somewhat different. This page
describes and illustrates some of the major differences in the
internals of the AR15 and M16.