Just wanted to point out that Bobby was wearing gloves when he shot the guy in the laundry room. I'm assuming loaded the bullets into the magazine with gloves as well. It's harder to get prints than they make it look in the movies and tv shows too, btw. If you've been fingerprinted before, you know how they do it to get a clean print, would be pretty hard to get an identifiable print beyond a doubt on something small like a 9mm casing. The surface needs to be clean before the print and if it's on top of a print or smudged or smeared in any way, good luck.

I would think the #1 reason they would leave the gun on the scene is because that's where the trail ends. Unless they destroy the gun (without getting caught first) there's going to be a trail back to them or back to someone who bought it from someone and so on. Leaving it there the cops have the gun so there isn't any chain of custody for them to trace back or attempt to trace back to who ever used it in a murder. It's much easier to match up bullets to a gun than it is to get a fingerprint or DNA evidence (other than semen) from a crime scene.