Say Scott - - have you tried round ball in your 12 bores.
: Years ago, I had an old "Fluid Steel" barreled double 12 bore with rather tight .725" bores. I shortened the barrels to 26" and installing sights on the rib. for balls, I used .715 balls and got 6" to 8" groups at 100yds with them, using smokeless loads as well as the heavy 12 bore BP load.(192gr. 1F) Jeff Tanner's mould .725" casts .722", so a .730" would cast around .728" for a more modern 12 bore. At $25.00 US$$ per mould, they're cheap to try.
: I picked up 12 bore WW wads at the trap range (red or white), cut the base wad off with side cutters, and would set one, cup down on the powder, then one cup up to hold the ball centred. If wadding was needed (for smokeless) extra fiber wads were placed betwen the bottom, cup down plastic wad and the cup-up plastic base wad. The press folded the crimp area over the ball just fine and stayed folded down on the ball. With BPowder loads, one needs a card wad down on the powder as the BP will melt the plastic.
: For smokeless powder charges, start around 24gr. SR 4756. That is a very low pressure load for 1-1/4 oz shot loads and I think I used around 26 to 28gr. for 1,500fps with a 545gr. ball. This gave the same velocity as the heavy African BP load, but kicked immensely less, just like a normal 12 bore load. That smokeless or BP load really dances the steel plates at 100 meters - and astonished a .300mag shooter as his rifle barely swung the plate. My 12 bore broke the 2X4 it was chanined to.