The clip on the m16 rifle they carry is a curved clip. The vietnam era m16 had a straight shorter clip that didn't hold as many rounds to prevent jamming. The clip on them in the movie are what are on the current m16 rifles.
the curved clips are not exclusive to present day m-16s. straight clips were used because they were more space friendly. not to prevent jamming. and vietnam era m-16s didnt only use straight clips. they used rounded clips too.
I tried to find info on the clips used in Vietnam after reading Matt's comment, and it looks as if the only ones used in the M-16 were the straight 20 round magazine. Also, the Army gave tips on how to use it (see link) http://www.i-kirk.info/tales/vnr08.html
I served in Vietnam with the 1st Infantry in 1968. Only the straight 20-round magazines were issued. The rounded ones may have been available late in the war, but I'm not sure. Remember, thought, this film is an allegory, not history.
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