You're talking hours of lost time because they now have to account for every single round. If I had ever come across a live round on a set, it would have been a huge 'oh shit' moment while the armorer(s) stopped work and checked and cleared every single gun and every bit of ammo (blanks, inerts, etc) to be sure there is no real ammo mixing around. You don't even need real live ammunition for a scene, so the only reason this happened was somebody broke multiple rules of weapons handling on set not to mention The 4 Rules. They would have hired security anyway, so it's an ok place to leave them. The father of Rust armorer Hannah Gutierrez-Reed, an industry veteran himself, gave cops a new theory for how live ammunition arrived on the set of the western film. not in Hollywood) the guns would likely have been left in the safe at the production site. You also don't take the guns off set until you're done with them. Might be a gun safe or just some other safe, but you're not going to force your way in. On every set I've ever been on, your 'somehow locked up' is an actual safe. Nobody but the armorer and their assistant(s) touch the guns, ever. Only after that, does the crew start working again to change scenes or adjust costumes, makeup, whatever. Then when the scene is done, the armorer takes it from the actor, checks and clears the gun and calls Cold Gun. The armorer (or whoever is weapons master for the film) should be the only one who touches a gun until they physically hand it to the actor. But you aren't wrong in your questioning why the armorer wasn't in possession of the gun. So, there are a couple ways this could happen.

I've never been an armorer on set, but I have worked prop crews with an armorer.

Rust armorer hannah