Ultimately, everything has to be made from scratch and ultimately, everything has to be bought new at least once. More to the point, however, just about anything can still be made pretty much as it was the first time it was every made, if someone wants it badly enough. Sometimes it seems like every firearm ever made is still being produced somewhere, from matchlocks on down, with all the gear to go with them. But some things are apparently harder to make than other things and consequently, they cost more than people are willing to pay. Still, people keep trying to bring back something or other all the time, like Lugers, for instance. Sooner or later, someone might make another New Service, maybe even a Python.
Given that firearms are "merely" machine shop products, it doesn't seem unreasonable to think that anyone sufficiently skilled in machine metal working should be able to produce any firearm, assuming plans were available (and assuming several other things, too). Just about everyone has made a copy of the Colt Government Model at one time or another. Of course, cost is always a factor.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.