This may sound like a corny suggestion but I have seen it on stage: The actor uses a prop pistol. He points it at the victim. Then the victim moves to block the actor holding the gun. At that particular moment, a sound effect of a pistol shot is produced. Hiding the gun from the audience the moment it is supposed to fire is only a matter of clever blocking. To be effective, the movements of the actors must be quick. If you perform in the round, the blocking must be extremely tight, and the moves quicker yet, then, bam! it's done. If you do it tastefully and cleverly, trust the audience to accept it on face value.