Amateur machinist here. I don't care about guns, so it would probably take a mistake or two, but I have zero doubt I could make a rifled barrel on the tooling down the block at my local makerspace. It isn't hard at all. (QC and repeatability are different topics, I'm not talking about commercial manufacturing.)

I can't do it in my garage, because I don't have one. My machines have to go up stairs in an urban walkup, so they're too small, and as mentioned, I have no interest in making guns anyway.

But yeah, a lot of people have both the skill and the means. Remember that people started making rifled barrels in the 1500s, a little while before we had CNC or overnight commercial-grade metal delivery.