>>63808529
That was in The Rock because it was kind of just a common thing in general. You can see such rifles in many movies, including with the soldiers near the end of Ghostbusters.
M16A1 handguards were a bit on the fragile side, and they were also separate left and right halves. Apparently the left ones liked to break more often than the right ones, meaning armorers typically always had spares for one side but never the other.
Thus, as the M16A2 rolls out, existing M16A1 rifles would then have their handguards swapped out with the new A2 ones if they broke theirs. Some also preferred them for ergonomical reasons.
Further, Colt were known to kinda just 'use up' parts they still had in inventory for commercial sporters, hence those can occasionally have varying parts. You can legit find some Colt SP1s out there which came from the factory with an M16A1 upper receiver, an earlier slabside or partially fenced lower, and M16A2 handguards.
