I have much of the tooling from an old gunsmith who died in the 80s. There are many rifling buttons he made for most any caliber. It is totally possible to make your own rifled barrels.
And there is no need for a CNC mill to make guns, a manual Bridgeport will do just fine. ;)
Electric discharge machining and Electro-Chemical Machining are within the range of a the home shop. I was curious and built an EDM borer once (I also built a wire EDM). I'm pretty sure pistol barrels can be bored with rifling in-situ using EDM under distilled de-ionized water. You will need a controller, which you can build for yourself. ECM is also quite feasible these days. Both are tech you can fabricate at home from ordinary materials.
Some seem to forget that back when the gun was invented there where no outlets to plug the CNC machine in to ;)