+2
So, so far nobody involved seems to be answering the must pungent question, which is why there would EVER be live, real ammo on a movie set. Blame's being passed around as to who saw the weapon last, who checked to see if it was OK, but the bottom line is that live ammo never appears on a set, not to mention a prop gun.
So, somebody put it there. Anybody with the slightest amount of sense would know to not do that. So, maybe some staff members were doing target practice with the gun before the scene and, again, nobody checked. The level of negligence here is almost astonishing. The only reason that I doubt some sort of murder mystery plot line is that, if someone deliberately loaded the gun, the would not have any way to know who would be the victim, so the British murder mystery plot line doesn't seem plausible.
What does seem plausible, however, is that someone wanted to wreck the production. A killing on the set just might do that, leaving so much disarray, controversy, legal complication that the movie goes down the toilet. It didn't sound like a very good movie anyway. It would not really matter who got shot, as long as somebody did and it was big news.
Look for either the Mafia, which didn't get paid off or a studio exec that didn't want to squander more money and wanted someone else to take the fall. When all the dust settles, there will probably be a movie about the movie, based on the book about the movie.