The heavier walled smooth 12's can be rifled with cutters. Buttons might not work. The other option is to have someone like "Moody" cut a barrel for you, then sweat it into your standing breech - if for the gun pictured. He can make any twist rate, octagonal, oct/round or straight round and tapered. He makes most bore sizes as well. I'd go with a 16 bore. I think his bore size is .665" & a 75" or 80" twist would be about perfect for round ball. The balls will weigh roughly 440gr. and if patched inside the plastic case, will give great power and accuracy. Moody's barels are for black powder only. He may be able to rifle an existing 12 bore barrel for you and if so, any twist. A .730" round ball will weigh well over 600gr. and is to be preferred to an slug in that calibre - for accuracy as well as killing power within iron sight range.