Martin- most of the rifled shorguns have a 36" twist. Were I to make one up for shooting round balls I'd like a slower twist, something like 60" or a bit slower yet.

A modern 2 3/4" plastic case has the capacity to drive 490gr. to 590gr. round balls (depending on the size & method of wadding) to just over 1,500fps with black powder or smokeless. Trouble is, a 12 bore barrel of modern steel in these twist rates is not generally available.

A viable possibility is to find a short, 26" barrel with a tight full choke. Then, the choke can be rifled as in a Paradox double. This should give rifling of .020" depth per side (40 'points' of choke total). A barrel with an 'extra full' designation should give a bit more depth, which would be even better.

A rifling bench can be made using a slow twist muzzleloading barrel as a guide. GM barrels in .50 and .54 cal. have 70" to 80" twists and would be perfect. The rifling guide runs through the muzzleoading barrel having a lead guide on that piece of rod that takes it's rifling in the rifled barrel. this rod then extends into the shotgun barrel with the cutter in the shotgun barrel. The lead plug in the muzzleloading barrel is guided by it's rifling and this turns the cutter in the shotgun barrel. The depth of rifling is controlled by the one who is rifling the barrel and should go to almost the depth of the 12 bore tube, perhaps a thousanth or 2 tighter. The muzzleloading guide barrel only have to be about 10" in length, and could even be a bit shorter.