I you want to put monster powder charges in that gun, get a straight octagon barrel, and have it rifles, in .62 caliber. Then you can fire as heavy a load as you can stand.
But, if you are going to shoot it with both a PRB, and shot, then go for the fowler, smoothbore, and don't even think about putting heavy charges in the barrel. They are not needed, BTW, as once you get that huge ball moving, it keeps on going. Most deer shot with a .600" RB will see the ball pass completely through the deer. The tremendous frontal area of the large ball disturbs substantial numbers of nerve endings un the skin, and as the ball flattens to .90 caliber, it destroys a lot of internal organs.
The lighter balanced fowler barrels make swinging the barrel on flying game, with shot loads much easier than if you have a full octagon barrel. How your gun balances of course will depend on other dimensions of the barrel, lock, and stock. The benefit of choosing a fowler is that it DOES give you the option of using a single gun to hunt both Deer, and upland game. Many are also used to hunt Turkey, and are successful at that task. As an all-around firearm, a flintlock fowler is about as versatile as you can get. :thumbsup: