On the point of practical play, obviously I don't want to burden players with minutiae. I'd like to explore the issue here and form a view, so I can present a consistent and practical approach.
Firstly, can we assume that a legal (or pretending to be) gun in 2073 has the following features:
1) Some sort of way of uniquely marking rounds fired. With that mark recorded on a database. i.e.
http://en.wikipedia.org/wiki/Ballistic_fingerprinting2) Some sort of unique ID
3) A history record of ownership from manufacture on a database
Let us explore a couple of cases:
Case A: black market gun.
The gun was brought on the black market. Imo, for the sake of practically, assume the gun has been 'cleaned' and given a fake ID. The cleaning will have removed the ballistic fingerprint and changed any physical and/or electronic serial numbers to match the new ID. Some hacker updates/inserts the ownership record, linking the gun to one of the runner's fake SINs with a licence. This deals with 1), 2) and 3). All part of the black market service.
If the SIN is burnt, he needs to get a new fake gun licence. When you buy a fake licence, re-cleaning and registering your guns is part of the service.
Case B: Gun brought retail with fake SIN with licence
This gun is perfect for issues 2) and 3), but any rounds left at crime scenes will be tracked to the SIN. The runner needs to have the ballistic fingerprinting removed. Once this is done, the gun this ready to take on a run. At that point it is effectively a black market gun.
However, in both cases, the runner cannot safety register the gun with more than one SIN. I am considering making this ruling in my game, unless someone can make a convincing case for how it could be done.
Perhaps the gun could have multiple IDs. However, I think changing a gun ID would be an extended test for a hacker.