As to old guns with defaced serial #s, there is a legal process for re-applying a serial number.
Generally speaking (and at this time, it could change) if you say "I have this, and I think it's not legal.." they will allow you to work the system to make it legal, without pressing charges. Sometimes there's no way, and the gun is forfeit, but again, usually without any charges, since its is obvious that by asking for help you have no criminal intent.
HOWEVER, the law is the law, as written, and they can charge you for posessing a gun with a defaced serial #, if they want to. It doesn't matter to the law who, when, or how the number got defaced.
I worked as a Small Arms repairman for the Army back in the 70s, and I can tell you from personal experience that in the shop, "parts is parts". One single gun being worked on always got the original parts put back in (as long as they were servicable). Two or three guns might get all the original parts back on the original frame, but there was no guarantee.
One maker's frame and another's slide mattered not at all to us. Neither did any accuracy standards. Barrels were checked for erosion, but that's all. Wear on parts only mattered if the gun failed to pass its final function check.
All else being equal (and it almost never is) bigger bullets tend to work better.