I looked in my Lyman Shotgun Reloading Manual for 20 gauge and 410 gauge buckshot. They used slowburning powders for the gauge/caliber and slightly exceeded the load levels used for conventional birdshot by a grain or two. The only major change they made was using a Remington plastic wad and removing the petals to make more room. Hand stacking the pellets is the only effective way to complete the load. #4 buckshot seems to be about the right size but larger will work, at a reduced pellet count. The 20 gauge uses #4 buckshot, about 20 pellets. The 410 used three 000 pellets! With these restrictions in mind, I'd try a weighted 3/4 oz of a particular size shot as my upper limit, use a slow powder from a list of reloads at the Alliant or Hodgdon websites. A Winchester WAA28 clone wad should do the trick, if you don't want to pay for the expensive Remington wads, just remove the petals from the wad before reloading and accept the leading as a price of doing business.

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Carson