I think they Defense will say the bullet has no connection to the murders, and in any event, it didn't come from RA's gun. Ejection scratch marks on a bullet can't really be irrefutably tied to a specific gun, in my opinion, no matter what LE claims. My opinion only.
But why argue it has no connection?
The defence position is the bullet is nothing to do with RA. The defence knows nothing about it.
IMO there is danger of going down the route of trying to argue that the bullet is somehow innocently at the scene, when that is scarcely credible. Better to argue that it doesn't connect RA to the crime because he isn't the killer?