Some places that are managed for waterfowl (but have other hunting opportunities available) require the use of nontoxic shot for ALL species....this includes small game such as rabbit, pheasant, woodcock, ect.
Steel shot is a no-no in a traditional muzzleloader unless you intend to use a plastic shot cup designed for steel shot (heavy paper, ect wont do). Cabela's had some double barrel shotguns that were steel shot safe for sale a few years back.....HOWEVER, I have found through a lot of trial and error and personal experience that steel shot will kill ducks, but only if the velocities are quite high....1350-1500 fps. I don't think that this is obtainable with black powder from a traditional firearm.
Bismuth and Tungsten-matrix (NOT tungesten-iron, which is as hard as steel) are the two materials that would be muzzleloader safe. Tungsten-matrix is not available as a reloading component in this country, so that leaves only Bismuth as a useful shot source for a nontoxic muzzleloading hunt. Prices for bismuth are high, but if you consider the number of shots you will be taking, it is still a viable alternative. I have found that in the smaller gauges of conventional arms (16 & 20 gauge), bismuth performs better than steel.
FWIW.