in the late 90s, I wrote to the RCMP asking what was the frame or receiver of a muzzle loader; was it the breach plug. I no longer have the written reply but it was the breach plug and perhaps the barrel. In a nutshell, if you build a pistol around an original breached barrel, the gun remains an antique because the rest of the parts are " spare parts". I also asked "how do you recognize an antique and the wishy washy answer was "you can usually tell by the design and construction. I wrote back and said there are a number of people today who build working replicas which are more or less indistinguishable from originals. I have forgotten the answer to that one other that it was ambiguous. In my writings, the RCMP put a lot of importance to the manufacturer's name on the barrel but the counter is that there are many guns with no maker's name on them and if of US or German origin, have no proof marks to indicate date.
I think if you start with a damascus barrel, that might qualify as a valid criteria for antiquity. I don't recall seeing damascus barreled handguns that were made after the mid 1860s. Having said that, sleeving an antique barrel and having it considered still an antique might be on shaky ground
cheers mooncoon