Tugg Speedman (Ben Stiller), along with various other characters, uses a 16" barreled AR-15 Sporter II Carbine throughout the film, which is anachronistic for Vietnam . The original AR-15 Carbine was an SP1 (sold between 1978-1985)), so there was no forward Assist on the upper receiver. Stiller's gun is the A1 version, or what Colt called the "Sporter II version" in U.S. sales. The Colt Model 653 based weapons came after 1973. He should be using an XM177 variation to be accurate to the era. It may be an homage to the movie Platoon since the characters of Barnes (Tom Berenger) and Elias (Wilem Dafoe) used the Phillipine export Colt Model 653 rifles. This film uses what is available in the United States, which happens to be the Sporter II carbine, since most of the movie was filmed in Los Angeles and Hawaii.
