Obviously, as previously mentioned, it would be your call.  It is, after all, your game.  My thoughts on the matter would be that he could bless some bullets.  However, it would be a long ritual (a full week of soaking in holy water, prayers, incense, what have you), would require special "ingredients" (such as the silver bullet idea mentioned, silver being another item of purity), would require the special markings previously mentioned and would definitely not be something he could throw together in a pinch.  Should he try this process with anything other than a bullet, I'd still make it a one use item.  He'd have to be a full fledged Saint or something and die using it valiantly or for the rest of his life for it to gain any lasting benefits.