As previously covered it depends on each individual barrel. I'd be inclined to go .020 less than bore diameter for round ball and try .005, .010, .015 and .020 thick patches - all available pre cut commercially. My .54 Green Mountain rifle barrel works well with .530 rb and .015 spit patch. The Curly Gostomski .72 smoothbore NW barrel uses a .715 rb and .015 spit patch or .010 with lube. Does an equally good job at 25 yds on soda cans or whitetail bucks. I suspect it is larger than .72 caliber and slightly smaller than 12 gauge though it shoots good with 12 gauge cards and wads. I'd want a rear sight on it before round ball hunting at 50 yds or beyond. Exceptional musket and smooth rifle (not an oxymoron but an actual two sighted firearm)shooters do surprisingly well on targets at 100 yds. I'd try shooting other owner's smoothbores first to determine not only caliber but fit (length of pull, drop) before investing in a new or used shoulder arm. Pillow ticking available locally mikes out to .018 thickness after it's been washed of sizing and dried. The kind of lube you use on your patching material will be a factor, too. I think Thompson Center made some single barrel smoothbores and Dixie catalog markets some single shot Italian reproductions such as a Mortimer rifle/smoothbore (exchangeable barrels) by Pedersoli. New ones aren't cheap. Good luck in your hunt for a smoothbore. They're a blast to shoot (pun intended). For birds I'd recommend #7 and 1/2 size shot for doves and quail and #6 for pheasants, #4 for turkey and ducks. #6 is a good compromise where you might have rabbits, pheasants, grouse, chukar, pigeons, prairie chicken or quail afield.
