It would not take much in the way of engineering know-how to fabricate a barrel out of steel bar-stock available at any machine ship.
A metal lathe, a drill press, a clamp and a micrometer are all easy to operate.
Being able to use a 3D printer to get the more involved bits would save some time.
The real issue as I see it is propellant. Making smokeless powder might be harder than making the weapon
Gun cotton is easily enough made. We can talk of that later.
Barrels:
Making barrels is part science, part witchcraft.
Drilling “deep” holes in a machine shop is one of the most difficult things to do. There are ways to do it on a lathe if you work at slowing down the feed rate to tenths per revolution. Harry Pope used to make barrels on a highly-modified South Bend lathe, so it can and has been done with good success. But when you talk of “deep hole drilling” in machining (where a “deep hole” is > 3 to 5 times the diameter of the hole), you’re into the really high-experience machinist skills - which mot machinists won’t have.
And that’s just drilling and reaming the barrels. We haven’t talked yet of rifling the barrels, which is a whole ‘nuther thing on top of getting a straight, uniform hole.
Now, if we’re talking of 5” pistol barrels, hey, we can do that. An aircraft drill bit that’s undersized, then ream to size, then just ram a rifling button through with an arbor press and you could have a relatively acceptable barrel.
But make a 24” barrel for a rifle in most machine shops? You’d better be looking for the guy in the back of the shop with the foul temper, white (or no) hair, of whom even the owner of the shop is scared. He’s the guy who probably can pull off the job.