I haven't been the least bit satisfied with steel in a ML. In modern guns the rule of thumb is supposedly to go up two shot sizes over what you do with lead. In my experience three sizes is more like it.
BUT! I say again, BUT!
You also have to get the velocity way up, on the order of 1500 fps to get anything like normal performance at normal waterfowl ranges.
Tried the three sizes larger in a ML, but was not getting very good penetration even at 25 yards. Killed a few, but crippled a few right along with it. In my experience there's just no way on earth to pump up the vels meaningfully with BP, at least while getting a pattern you can hit with.
The alternative varieties to steel may seem expensive, but look closer. Shot for shot, loads from a ML are bunches cheaper than loads for modern guns. And my busiest day ever in a duck blind was 13 shots. While with a modern gun I might take 2 or 3 shots per flight, with a ML SxS I take one and save one for cripples. And I seem to kill a lot more birds in spite of shooting about 1/3 as many times.
At the current $154 per 7 pound bag, that's right at 90 shots of 1 1/4 oz each, or right at $1.70 per shot. That's about half or less what premium 12 gauge shells are going for. A gunner has to be pretty busy AND a pretty poor shot to use up more than a bag a season. I know guys that can easily bust $100 a day with modern guns.