My .54 smoothbore is a 15/16" straight octagon 32" long, and made by GM, mounted in a Lyman GPR stock, so this is a strong and heavy barreled smooth rifle, and not at all like many of the fine graceful light weight bird guns a lot of the guys here have, so please consider that before using my loads, as they may or may not be safe in your gun. And you didn't post the details of your gun so I am just listing what I use. Hope it helps.
28ga:
60gr FFFg, 2 cardboard OP cards, 1 oz #6 shot, 1 OS card
70gr FFFg, 1 cardboard OP card, 6" jute cord frayed into a ball of fiber for use as TOW, 1 more cardboard OP card, 1 7/8 oz #6 shot, 1 OS card
70 gr FFFg, 2 leather OP wad/cards, patched .527 RB or .515 PRB, with different patch thicknesses of course.