Most modern blank guns are designed such that real rounds won't chamber.
Yeah, that is a curiosity to me. I don't know anything about the Hollywood movie set firearm policies, but it only seems sane to have "firearms" that absolutely cannot accept a projectile cartridge of any kind.
Obviously, as Hank pointed out with the Hexum death, a blank can still kill, but with the state of special effects today, I, as a layman, can't see any valid reason for a real gun on a movie set. Especially when most in Hollywood hate guns, you would think there would have been a push for something like this. But then guns are cool and okay in movies, I guess.