Speers loading manuals list load data for their capsules.

I've found that you can actually get more shot into a case by not using the capsules and loading it the way Capt. Cormorant linked to. The thick base plug, capsule thickness and seating depth outweigh the exposed parts capacity.

There is a local bullet caster that used to carry a whole line of shotgun loading supplies, unfortunately now they are hit and miss on what they have on hand, but they did carry both no. 12 and "dust" shot.

I used to make some full length shells for my .41s using cut down .30-30 brass. They held a lot more shot.