In many scenes, the flat 'slab side' upper receiver of the original M16 (AR-15/SP1) rifle is seen. Some of them appear to be genuine civilian AR-15/SP1s and aren't seen fired full-auto. Instead, M16A1s seem to be used to depict full-auto fire. That is, until the Grenada sequence, when the Recon platoon's weapons are both M16A1s and M16 rifles, possibly AR-15/SP1's converted to fire full-auto, perhaps to to a lack of enough M16A1 rifles for those scenes.
What's the language limits of this site? Just to be sure.