This is possible but the monumental stupidity (and evil) of such a criminal act (with intent to harm) is off the charts. I am guessing you are implying a disgruntled union crew did that before they all left 6 hours before the shooting, so timing is also another issue with that theory.

In any event, they have the shell (with possible fingerprints), and prints on the guy as well. 3 people are supposed to have touched the gun (armorer, ad, and actor), and only 1 person's prints should be on the shell, the armorer. No doubt forensics will be important in this investigation.