Top-not, I'd hit up Jeff Tanner to make me a mould to case .740" ball. The reason for that size is a .740" ball will allow a nice thick patch which will hold lots of lube and be easy loading and shooting. To cast at .740", the mould will have to be a about .743", thus casting .001" to .0015" smaller per size due to shrinkage, they'd come out at .740". If you had to use hard lead, you could - and still use the thick patch.
A .005" or .010" undersize (smaller than the bore) ball is for a rifled barrel, which has another .010" to .016" of grooves for patching to collect. the smoothbore has no such grooves, thus the ball must break the .005" or .010" 'rule'.
If you want to shoot naked balls, your accuracy will improve with larger balls, or with a smaller one, a double wrap of appropriate paper in a military-type paper ctg. BUT - If using paper ctgs. they must be V/TIGHT or you will promote fires due to powder flame passing the paper and stating it on fire. We've proven to OUR satisfaction, that in rifle or smoothbore, if the paper is tight to the bore, there is no burning of the paper upon discharge.
As for paper, printer paper is available in a remarkable range of 'weights'.
A pure or dead soft .740" lead ball will be just over 600gr. weight.
As for powder charge, your shoulder will probably stop you before pressure becomes a consideration. I would think anything up to about 130gr. would be more than sufficient. You will want to shoot it on paper, right, then left, with each charge to see how each barrel prints in comparrison to each other.
I would consider putting both tubes into a 4" to 5" group at 50 yards to be wonderful shooting, with a double smoothbore. We were able to get one of LB's smoothbores shooting 3" at 50 yards off bags. Mine won't do that.
