This might be making more sense. If the union guys walked out and the non union came in, who is to say, or not say, that a union hand might have slipped some live ammo onto the set. Union hands can get quite serious.
I have to admit that I don’t understand how a live round can get into a prop gun. Then again, it’s technically possible, but it’s not plausible.
The safest gun movies are sci-fi movies that use lasers and phasers. None of those actually have “live ammo”. 😜