Apparently, there was a replica gun on set. Namely, there was the gun that Baldwin had in his hand that killed Hutchins, which clearly was a real gun, and there was a gun that looked identical while being inert.
First of all, this is a perfect example of why you should never talk to the cops without a lawyer. Baldwin could have plausible deniability if he hadn't told the police -- in fact, the world -- that he was aware that he was holding a real gun.
Second of all, this seems like a bad idea in regard to dealing with guns on movie sets.
I know that we've had real guns and fake guns on movie sets (almost) for as long as we've had movies. Still, apparently Baldwin's gun was pretty specific; so, the inert replica was probably made to be passable for high-def close-ups.
This seems like a mix-up waiting to happen.
If you're gonna have a real gun on set, just keep using the real gun so you never convince yourself that you don't have to be careful.