They may not be able to tell who the shooter is from GSR, but DM had pics of the gun on his computer dated prior to MS, and DM ended up with the gun after the crime (until he was arrested and MS sought to sell or hide it - on DM's behalf?)
In my mind it is DM's gun, he used it, and he was the shooter.
Plus DM had the satchel. GSR found there was well.