Pyrodex is made with Potassium Percholates & so is T7.
The fouling is more hydroscopic than black powder fouling. The sub's fouling converts to a BAD acid when moisture or high humidity is present - much worse than damp BP fouling.
It is almost impossilbe to get Pyrodex fouling out of pits - with solvents, BP or Smokeless - it will keep eating and eating away in the bottom & sides of the pits.

Please use only real black powder in that beautiful shotgun.

Kaimiloa's load using hard 1/8" cards along with the woven felt OxYoke wads is indeed a good one. I've never used them in a shotgun myself, only in BP ctg. rifles where they excell. They would also excell in a 'pelter'. Don't worry about the thin "B" wad hurting patterns, They spin off away from the shot cloud almost instantly upon exiting the barrel. Photographs show this.

Good luck with your Harkom.