Jeff Tanner is also a great source for round ball moulds in any diameter. But I am nearly certain you can get some premade brenneke style 20 gauge slugs too that I would guess might work. Brenneke style work well in my SMOOTHIE and RIFLED barreled slug guns.

I would also stay with the round ball shooting. Did you try those RB's in an UPSIDE DOWN shot cup with the petals cut off?

Trying various powder amounts and types of powder also has a big role in helping to get the loads to point of aim with a double, in my experience.

My round ball shooting seems prefer slightly slower burning powder but alot of it! Boom! Boom!