Dropping the gun at the scene is SOP in a Mafia hit. He was wearing gloves so they won't get prints off it. Better to drop it right there than to get caught with it in your pants a block away
DNA off the shirt? Lots of hoops there. For one thing, there has to be a sample of Bobby's DNA on file somewhere in the world for it to come back to haunt him. The shooting was in Canada, wasn't it? Why would the Mounties think that it was a NJ based mob hit?
Now, if the bling is inscribed, "To Bobby from Janice, all my love" or some crap like that, maybe he's got a problem. Presumably, he had to show ID to get into Canada, his last name would be well known to the Feds who'd cooperate with the Mounties, and it'd be kind of hard to explain away as a coincidence that a guy married to Tony's sister, a known member of the mob, entered and left Canada the same day as a stiff turned up holding a chain with the first name of the known mobster and the boss' sister on it.