At least as useful as it would be to a normal person. If you defined a certain wizard, like Doctor Strange or Harry Potter, I could better answer this question, but I’ll try regardless. Let’s use the power of telekinesis, which seems to be the hallmark of every fictional wizard ever. Combined with the use of telekinesis, a shotgun could be a deadly and unpredictable weapon. If a wizard had enough control over his power, he could send bullets flying in many directions. He could even arrest the bullets momentum, place it in an unpredictable location, and send it flying in the back of an opponents skull (I am assuming this is a combat situation. Because I feel like it). If the opponent is a wizard of equal strength, however, this can be a disadvantage. If the opponent wizard is as aware of his own powers as your wizard is, he can turn the tables of the battle. I recommend (if you are asking this to write a story) making the bullets both a strength and a liability of the wizard. He can use firearms to considerable effect, but he needs to be constantly aware of the fact that other wizards are just as powerful and can do the same to him.
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