The correct answer is you would treat both guns as real. Even if they were both replicas, they appear real enough, so you would be justified in taking defensive action, including deadly force if someone were to point the gun at you.
As for Alec Baldwin, all legal arguments I've heard say the legal onus was on the person who was handling the firearm. He will not face any charges for this tragedy. Now, as for a civil suit, he, as a producer on the film, definitely faces legal challenges. He obviously didn't hire competent people to handle the firearms, and the fact the first crew quit because of working conditions is troubling....