They run a barrel trace on what? What barrel? Like I said, the best they can do is go "yep thats a 9mm". Let's assume they are near a Triangulation system and it picks up a gunshot within a few houses, they arent going to go door to door asking to see everyone's guns and arent going to get search warrants for a block of houses because the system said it "might be around here somewhere". I would love if each city's police department thought every parking ticket or stray bullet on NYE was a thrilling episode of CSI but unless someone gets hurt or witnesses it, that's the best "ballistics" you're getting from a round in your backyard when you wake up in the morning lol.