Dismantle it????
He's not stripping it down to clean the thing - he just needs to know if there are any rounds (whether blank or live) present and look down the breech into the barrel.
Tell me you're clueless about firearms without telling me you're clueless about firearms.
Perhaps he could have briefly inspected it and ascertained for himself what state it was in, it's not exactly rocket science and it can be done rather quickly.
It wasn't though, it was picked up by the assistant director (not the prop/weapons person) among three weapons placed outside the set and handed to him and it's clearly not stupid. What's stupid is the naive
yeah, just assume it's safe, don't check for yourself position you're supporting and which has ended up up with one person dead and another injured.
To try and claim otherwise is mind-boggling tbh...