I used to keep up with this issue. Last thing I recall reading on the topic is that the supplier of the guns and supposedly dummy ammo is the one who mistakenly put live rounds in the order which was sent to the movie set.
Dummy bullets look exactly like real bullets. Why no charges against the supplier?
Maybe they know something I don't.
The live ammo came from crew members who acquired it for recreational target practice after hours. It is possible that the Armorer was involved in ordering that supply. The exact person who ordered the live ammunition has not been publicly identified. The crew members who participated in the target practice have not been publicly identified.
To be sure, neither of those activities was a crime. They were just stupid risks under the circumstances. Stupid risks accumulate.
The presence and unsupervised handling of live rounds on set was one of the safety issues that caused many of the previous crew members to quit.