I believe many muzzle loader barrels are made of softer steel like 1157.
They are designed for Black powder only and I believe that is what the barrelmaker would tell you.
The groove depth is .010.That is for a cloth patch.Typical groove depth for unpatched bullet rifling might be .003 or .004.
As Hawg and others have mentioned,1 in 56 will work great for a patched round ball but it will not stabilize an elongated bullet.
Whatever you are planning,you will still invest all the work and other parts,like stock wood.
Look in Brownells,at Green Mountain,and E R Shaw if you want a moderate priced octagon barrel.You also might check with Buffalo Arms.