I have a Greener Martini in a 12 bore rifle. I believe most available barrels will run around .736 or so- you will have to determine that when you get it in hand. Bear in mind that chambers for brass shells and paper shells can be of different dimensions! Whether you use brass or paper may depend on the actual bore diameter. In other words, a larger bore will be good with brass, where a tighter bore may need paper or plastic to hold the smaller ball correctly.

My Greener needs paper, I have a Lang double 12 rifle for brass, and the paradoxes all need paper.

So- my advise is to get the barrel first, determine what size ball it will require, then decide what shell to use that fits the ball best.

My 2 cents-- Doug