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In the Eighties I was able to watch a low budget film being made. There were a lot of guns, all real weapons where in individual cases and marked. Only one man handled the real guns. The fake M-16s were very realistic. However up close it was obvious they were incapable of firing even blanks. The upper and lower receiver were solid pieces of metal that did not separate. They looked and felt like the real thing. I gathered they were expensive because they where in a large crate with each weapon separated by foam rubber. When they filmed the big shootout, the people who died in an ambush were filmed dying first. Later, they filmed the actual shooting parts from an ambush and we all had to clear the set because of the blanks. That on the set of a really low budget martial arts film.
If that’s an indication of how guns are treated on set, it’s no surprise that this “accident” occurred.