The bore and groove diameters will be determined by the size of your shells and the inside diameter of the shells. You’ll want the ball to be .010" over groove diameter, but that ball has to go inside a shell. The grooves should be about .008" deep.
Let look at a 12 bore as an example. Let’s say you have some paper shells that will take a .720 ball. The ball will have to be .101" over GROOVE diameter, so the barrel would have to be .710 at the grooves. The lands come up .008" all around, so that equals .016" total which is subtracted from .710. That means your bore would be .694" before rifling, and .710" in the grooves after rifling
To build a gun correctly you have to get all the math right going into the job.

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You can use any good rifled barrels you like IF you can then make your own shells. The same formula works. You simply make a ball .010" over groove diameter, then make your shells to accept that ball, and then make a reamer to cut your chamber to fit the shells you made. The reamer should be about .004" over shell size to allow for expansion and contraction when the gun is fired.