How/where was the gun recovered ?
Though I do not recall at the moment the exact news source I'm basing the following on; as I understand it, the police searched local area pawn shops, found a possible match (the right caliber, and maybe twist?) then went back to the pawnshop and retrieved that weapon for further testing.
Yet I don't trust the testing to be all that conclusive. It's more like a "this could be a possible match" than anything approaching a "this is the exact weapon, and could in no possible way be any other weapon of the same general make, or even similarity to weapons made by yet other manufacturers that use near identical tooling.
The reality is that the it's not like television detective shows in "tv drama" fashion, portray the truth to be, about how good ballistic forensics can be, when it comes to matching precise projectiles having been fired by an exactly specific weapon.
It can be worse than bad fingerprinting/identification -- which we also have a lot of in this nation.