Bad idea. (1) he may only get one shot to save his life. (2) If his blank doesn't do the trick he then has to drop the mag, find and insert the mag loaded with live rounds, cycle the slide then reaquire the target. I hope he has the time and steady nerves for all that. OTOH, it's his life, if he wants to just yell "BANG" to scare them it won't affect me either. BTW, blanks won't cycle the slide.
All that is neccessary for evil to triumph is for good men to do nothing.