It doesn't have to look like a handgun, and the noise doesn't matter. My guy is going to shoot a couple of scummy drug dealers who have taken a kid hostage. He happens to be a trained sniper, and happens to have a Remington 700 in .308 with a variable scope lying around.

He shoots these guys, drags them into their house and sets the house on fire. (This is in the middle of the woods, no issues with anyone happening to come across him while it's going down.) I was going to have it come back to bite him, in that when the fire is finally put out and the investigators find the bodies, they find the rifle bullets in the bodies. I was kind of thinking the fire might have gone out, or might not have burned the bodies completely, just charred them. I'm also assuming that even if they can tell they were rifle bullets, they can't really get any ballistics information. He throws the rifle away anyway, so that doesn't matter.
He's using hollow points to avoid blow-through. How far back does he need to be for that? He's a pretty good shot, but this is all happening in a hurry, no time to wait for the target.