During the movie, the bad guys (and later out hero) fires an M-16 rifle which contains blank rounds. Manny firearms (including the M-16, M-60 and SLR) cannot fire on auto or semi-auto when using blanks. They can only fire once and then must be manually recocked.

The Reason:

When one of these weapons fires a bullet, expanding gases push the bullet out the barrel. As an opposite reaction, the gas also recocks the weapon. Without a bullet, there is nothing for the bullet to push against, so the next bullet is not loaded. To get around this, soldiers place a BFD (Blank Firing Device) on the end of the barrel (or replace the barrel for a M-60). It is quite obvious if one of these has been fitted. I presume that the M-16s used in movies actually have special barrels.