The professional killer weapon of choice is and was a stolen or "ghost" gun they drop at the scene without any prints. The old Cosa Nostra special, because the real concern is that you're caught with the murder weapon; it doesn't matter if you were quiet if you have an single-purpose assassination tool after all.
So the cops find a gun they can't trace, a dead body, no other physical evidence and frankly that sort of murder isn't typically solved.
So the cops find a gun they can't trace, a dead body, no other physical evidence and frankly that sort of murder isn't typically solved.