I suspect anyone with a rifling machine could rifle a full or extra full choke. That would give .020" depth per side on a full choked gun. It would not be difficult and the rate of twist would be in the 60" to 70" range, just fine for round balls. Given consistant bores, a cutter head would not be difficult to make and one cold use an old muzzlelaoding rifle barrel section for a guide. Pretty easy for a handy guy with a minimal shop.
: There are a at least couple guys over at americanmongrifles.com who have rifling machines. One's machine is for sale.