Weaponsman lays down on making/adapting rifling machines.  It appears easier than I was seeing the last time I looked at it, but still complicated, especially if you want to make lots of barrels.  You still need to gun drill though, and that’s not fun.

Drills and the difficulty of drilling a hole is generally determined by the ratio of the diameter to the depth.  Per a manufacturer, gun drilling is typically used when the ratio exceeds 20:1, and can work up to 400:1.  For a .223″ hole, that would be 4.46″ to 89.2″  I doubt many people will be putting an 89″ barrel on an AR, so you can see the advantage.  It uses a special straight flute bit, not all lathes can feed slow enough to run one, and you need a high pressure coolant feed.

Be wonderful to get a post by someone who’s done it, to get a proper laydown of what’s needed.