From a pretty detailed story I read on it he hired a non union company to do his prop work. They were filming in Mexico so it's probably outside the realm of regulations required by unions in the US who perform the same jobs. Even at that those there's a industry standard whereas guns used as props are never pointed at the actors they just make it look like they are. The prop guns also are designed not to use real bullets. There are also extra safety protocols in the US when using prop guns in movies, they have to be tested and signed off on then there's individual instructions with the actors about safety protocol which also have to be signed off on.