I think the answer to your question might lay with your observation that it is a serious matter.
Evidently, the ammunition in question might be from one of the following "flavors":
A. Packaged and marketed for movie use by a company supplying blank, dummy and perhaps live rounds.
B. Be generic ammunition purchased from a gun store, gun show or sporting goods store.
If its an "A", I imagine that only a few people other than the armorer would have been able to bring it to the set. But.... if its a "B", then a far larger number of people could have brought it to the set. There may have been multiple people.
My guess is that the investigators might be up against a Middle School Wall of Confusion if the ammunition is "B":
- Oh heck no, I never brought live ammo to the set. I did, however, target shoot with some live rounds from an open box that somebody else brought. No, no, I have no idea who that person was.
- No- I never, ever brought live ammo to the set. It was in my car- never at the set. Anyways, I shot it all during a target shoot and that was the end of it.
- Ok, I was seen with some live ammo, but I did not bring it to the set. Rather, "D" gave it to me on the set.
- "D": No, I did not bring it, I got it from a table, people were saying that it was extra and we could target shoot.
- Yes, I told people they could target shoot with extra ammunition, but I did not bring any ammunition to the set. Somebody else did.