Guys - Lyman's 5th Shotshell Handloading manual will give some ideas of where to start, perhaps, but maybe not so much with the really heavy bullets. I have the 3rd, and found it to be measureably helpful in my loads, even with the 16 bore, which they only load with buckshot.(in the back slug and buckshot loading section) The 12 and 20 bore both have buck and slug/round ball data which is useful.

Much depends on what level of power you are looking for. consider the 12 bore Paradox guns were normally loaded with around 750gr. bullets running 1,000fps to 1,200fps. That is a 1.7oz. slug sifting along at barely the speed of sound.

Note also, the big, short slugs and round balls are not effected by the drop down through the speed of sound like small diameter long pointy bullets are. For proof of this, I look no further than my .69 cal. rifle and it's round balls. If it can start a ball at 1,550fps, and still deliver 1.2" 5 shot groups at 100 meters and a 6 shot 1 1/4" X 3 1/2" group at 200yards with me and my 60 year old eyesight running the Express Sights, then the speed of sound is not impressing the ball whatsoever. Starting at 1,550fps, and with a BC of .090, the ball drops below the speed of sound around 100yards (2,000ft. ASL) ie: 1,077fps, then is only trucking at 877fps at 200.

Also- a PM to Ed Huble might give you a place to start with slower smokeless loads in the 20 bore with short cases. He helped me get started with slow powders for my 12 bore Mossberg rifled gun.