I would guess that you get a piece of good steel and turn it on some kind of computerized metal lathe to get the shape you want, and then you drill a hole in it. Or maybe you drill the hole in the block of steel first. How do you get the riflings in? What about chrome lining, how is that done?
What is the difference in production for a cheap AK barrel versus a barrel for a $5000 sniper rifle?
Is a shotgun barrel with much thinner metal and a bigger bore made the same way?