Marines in both Elias and Barnes' squads use M16A1s, most commonly seen in the hands of Pvt. Chris Taylor (Charlie Sheen).
MGC M16 replica
In several scenes, MGC-16 replica rifle in place of blank firing M16A1 rifles. They are noted by their shortened forward assists. Most MGCs have blued magazines but they appear to have regular mags stuck in instead.
M16
Crawford (Chris Pedersen) is briefly seen using what appears to be an original M16 as it lacks a forward assist. It could also be an AR-15/SP1 rifle, but since he is not seen firing it, it is unknown.
What does it mean when the forward assist is rounded? Modern update?
M1911A1
The M1911A1 was the standard sidearms for all of the NCOs and officers. It is most prominantly seen used by Sgt. Elias (Willem Dafoe) when he does tunnel rat duty and Sgt. Barnes (Tom Berenger) when he uses it to threaten a little girl in the Village to get her father to talk.
In the movie Platoon, the NVA soldiers used the Chinese Type 56, their copy of the Russian AK-47. In the real war, North Vietnamese used many real Russian and Chinese AKs, with the vast majority of supplied weapons coming from the Chinese. The Chinese Type 56 AK was also the rifle that Chris Taylor, played by Charlie Sheen, uses to shoot Sgt. Barnes.