I'm out of touch with the newer shot in a ML, but up till we had to quit using lead here in Alaska (later than other states, but I don't remember when), I used a Navy Arms 12 for most of my waterfowl hunting. I'm guessing that just as makeumsmoke sezz, you're going to have trouble getting enough velocity with BP to make steel shot effective. In my cartridge guns steel is only effective when you really crank up the velocity, something you're just not going to do with black.
Not that it does you much good other than to provide food for thought, but my lead shot loads chronoed just shy of 1100 fps, and I always opted for a little bigger shot than I used in my faster cartridge guns, even with lead. I used #6 in my rear stuffers early in the season and #5s in my front stuffer. Late in the season I went to #4s in the rear loader and #2s in the front loaders.
With lead I never felt the need for heavy shot charges, with 1 1/8 ounce being the right compromise for velocity versus recoil. Patterns were decent even without plastic cups, but easily achieved a mod choke equivalent with WW Red wads. Less with the white.
It was roughly a 35 yard gun with the cups, but with no wad I was stretching things once I got out to 25 yards using only 1 1/8 ounce of shot without a cup. I just moved the decoys closer and picked my shots, usually reserving the second shot in case of wounded birds. If I missed on the first, so be it. I wouldn't crack off the second.
Don't know if that helps any when using today's components, but if you can sort through the development and swallow the high cost of shot today, you're on the right track for the most satisfying waterfowl hunting ever.
