M16A4 Rifle. Almost the exact one used in the movie.

The M16A4 rifle is carried by various Marines fitted with ACOG scopes (both TA31 and TA31RCO models), RIS foregrips, Surefire Universal weaponlights, and AN/PEQ-15 IR designators. Soldiers of the 40th Infantry Division (CA ARNG) are also armed with M16A4's, but without the An/PEQ-15s. Joe Rincon is seen using the M16A4 as well to defend a Marine while he was treating another wounded Marine.