On the Apocalypse Now page, I have 9 M16A1 images. It doesn't look like too many, at least to me.
http://www.imfdb.org/index.php?title=Apocalypse_Now
As long as there are different actors handling the same gun more than 5 seems ok., but no more than 10.
Oliveira, not to bash your work on We Were Soldiers. But 24 XM16E1 images looks way overboard.