Frist off, it is also the actors' job to ensure the gun is safe. He should have been there when the gun was checked and verified it for himself especially when he purposely hired a fuck around and find out armourer.
Secondly, how were they supposed to know your level of knowledge about firearms and ammunition? With them explaining stuff in a simple and quick manner, we are all now operating on the same level of basic knowledge about this, so there should not be any miscommunications going on.