No black powder or substitute allowed in registered shoots but. 82grains of ffg is 3 drams. Do the measure volume with that much bp, then whatever that volume is it is what you use with pyrodex. Wads? You’ll have to trial and error the wad for stack height.

 

Pyrodex is more corrosive than real black powder. Clean with water, then use a moisture displacing product like WD-40.
I shoot BP Trap all summer.

 

What fg of powder is used for a 12 gauge shell? Would ffffg powder work?

 

Definitely not. 4F black is priming for a flinter.

I use 80 grains FFg Goex in my BP shells, over powder card, two 1/2 inch cushion wads, and the same measure of shot - about 1 1/8 ounce.
Rather than using lubed wads, we put a dab of Crisco on the crimp, to keep the fouling soft.
I use Federal Top Gun hulls, the old kind with paper base wad.

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For 12 ga BP shotguns, I use 75 grains (by volume)of Pyrodex RS or same amount of FFg Goex / over powder wad, 2 felt wads, 1 1/8 oz shot and an over shot card. If it's a cartridge gun, I use Magtec brass shells with large pistol primer. Same load in the 14 ga. English fowler. For 16 ga. ( muzzle loader) I reduce the shot to about 1 oz. The beauty of muzzle loaders is that stack height isn't a problem so I often use only 1 felt wad. Wonder lube on all felts.

 

Use 1F, 2F, or 3F, but leave the 4F for priming flintlocks. The more Fs, the finer the powder, the more pressure you'll generate. When I do use BP in my old SxSs [ 12ga ] for clay birds it's usually 1oz with 70 to 75grs of 3F or 1 1/8oz and 82 of 2F. AS said, you have to clean the same day as shooting whether it's BP or Pyrodex. Use only fiber wads, no plastic. It'll melt in the barrel and is just alfull to remove. Graf's sells BP pretty reasonable, and I believe fiber wads. You['ll need a overpowder card - .125, some soft cushion wads - .500, and overshot cards - .028. The cushion wads can be cut for proper stack height with you thumb nail, or knife. Good luck.