I know you were playing Devils Advocate and I hope I didn't come off as know it all-ingly.
I have crash-course researched bullet extraction marks and learned that 2 different Sigs can/will have different extraction markings, especially based on the length of time owned and number of rounds that have been shot. Since they recovered his Sig during the search of his home, I have no doubt that ejected a ballistic test to see if those marking do indeed match to the bullet recovered from the scene.
I think the bullet will be just one piece of a much bigger picture that indisputably points to RA as BG. He's already admitted much of the other pieces himself.
JMO
Yes
It is conceivable that there is another gun out there that also is a close match - but that is not the point.
We have to see all the circumstantial evidence together.