The modified chokes of a single or double shotgun could easily be rifled, using a muzzleloading-type rifling machine. I expect to do just this, this coming summer with an old gunsmith friend of mine from Kelona, B.C. He's rifled a few barrels in the past by hand and feels it a simple job to rifle a shotgun's choke, like the Paradox guns of the 19th century. I agree. A square twisted rod of correct twist rate, held in a yoke is all that's necessary to turn the cutter - much the same fashion as used in the 19th century. Time and energy is required, of course.
: I suggest improved modified as that will give .010" depth per side in a standard 12 of .730", whereas full choke would leave .020" per side due to it's 40 points of choke.
: I do not know the measurements of chokes in more modern 'over bored' shotguns wiht .735" to .740" bores.
: The bores shown above appear to have very deep rifling indeed - fully .015" anyway - maybe .020". Perhaps a full choke is the proper one to rifle.
: Any modern shotgun should handle 1-1/2 ounce of lead at up to 1,400fps. 1-1/2 oz. is 650gr. Incidently, if cloth patched in the case, slightly undersize solid round balls should work to 1,600fps, a full 100fps faster than the heaviest 12 bore load of the 1870's and 80's. For them, it took 190gr. black powder. For us, it's about 26gr. (just a guess - not loading data) of Hodgdon's Longshot, shotgun powder.