Either in a lathe, or by doing this:
This is a picture from my own file - my gun, my hand, my messy bench - hand-cuff pouch hanging on a nail between the file card and square - LOL. Rotate the wrist, rotate the barrel now and then. Should be done from your machined 'crown' in about 45 seconds.
The thinnest patches I normally use, compress with caliper tines squeezed between forefinger and thumb as hard as I can, comes in at .021". Non-squeezed thickness is immaterial/of no use.