I spoke with Rice about the possibility of making Forsyth-type rifled barrels. They seemed interested. This was a year ago last July. You could try them.
Forsyth-type barrels are of large calibre, with shallow, very slow twists. The slow twist allows very shallow (for around ball gun) to be made. They 'could' be button'd, but probably are cut rifling. I'd think a large calibre button would be too costly. buttons usualy are about done at .004", however some have been made that way to .005" with reasonable results. Deeper is not really possible and leave a good inner surface without tight and loose spots - wonky in appearance to the eye and can be felt. It's been tried in smaller bores, with poor results. I know, I've had a couple.
Ed Rayle will make large calibre barrels of your choice in size and rifling twist - cut rifled. His 16 bore barrel is actually .670" or .675" to allow a .662" ball, a common size made by Lyman. .675" is actually a tight (by .002") 15 bore.
I should add that a rifling twist in a large bore would answer if around 80" to 90" rate of twist. Any more would take more powder for best accuracy, than a person might like to use. My own .69 (14 bore) has a 66" twist, but is not happy unless it's fet a LOT of powder. It's accuracy just gets better the more I dump into it, right to 200gr. 2F. I do not use more than 165gr. due to recoil and obtaining descent velocity at that level for my use. A 100" twist 14 bore might need more powder than a person would like shooting. Dan P. has a 16 bore Rayle with, I think, around 75" or 80" of twist IIRC and it shoots well with a little 140gr. Swiss charge. That's about equivalent to 160gr. of GOEX.
These calibres are not everyone, but make the very best hunting rifles there are - for any game. 12 bore is a big big, as velocity suffers. I've a friend with a .75 (11 bore) and even he feels it's a bit too big due to trajectory with useable loads.
A 15 bore ball weighs 466gr. - for a 14 bore rifle. I shoot a .684" ball in mine- at 480gr.
A 16 bore ball for the Rayle barrel at .662" weighs 436gr. I peersonally think this would be the ultimate size for any North American big game hunting & would do for all but perhaps Rhino, cape Buffalo and Elephant in African game.
The 14 bore, shooting a 15 bore ball was one of the more popular Indian calibres for all game including tiger, buffalo and elephant. The 16 was figured to be the minimum calibre for those animals. This, according to Forsyth himself.
Pac-Nor makes 20 bore and/or 12 bore rifled barrels but uses too fast a twist for a muzzleloading round ball barrel.