Post #9, regarding the 20 gauge being legal to use in all 50 states, refers to the minimum legal requirements for bore diameter in a centerfire shotgun for turkey hunting.
As far as I know, no state currently has a greater standard for bore diameter as regards to a muzzleloading shotgun/smoothbore, than they do for a centerfire shotgun.
As a result, if you purchase a 20 gauge muzzleloading smoothbore, then it will be legal to hunt with regardless of whether you hunt turkeys during a muzzleloading season, or the regular centerfire season.
20 gauge is usually the minimum bore size for waterfowl in most states, although I do not know if that is applicable for all 50 states.