This isn't the same question as posted a few minutes ago!
I have a Ruger 10/22, I have ordered a replacement barrel, which has been shipped to my local RFD. I was intending to give the RFD my old barrel (for destruction or whatever) then take the new one. When I was there the RFD said that as Ruger 10/22s don't have serial numbers on the barrels, it might be possible that I could keep the old barrel. He called the local firearms licensing team who said that even if I want to dispose of the old barrel, I'll need a one-for-one variation for the new barrel. I know of several people that haven't had a variation to do exactly the same thing, so I was a bit surprised. After this variation I presume I will end up with a FAC with exactly the same 10/22 with the same serial number (as it is on the identical action).