Story time...
I am 64 and have shot nothing but lead during my wing-shooting years which ended about 1985 in Alaska where I lived for 35 years shooting ducks, geese, and brant. I just lost interest and waterfowl seasons there were generally very cold and wet.
For pheasant in Nebraska (before I enlisted in the USAF in 1971), depending upon the terrain (Nebraska can be hilly or flat) I used a Rem 870 Wingmaster 26" 12 gauge I/C choke with #6 for the flatlands. For the cornfields on hills, where long-range shots are more the norm, I used my Dad's Win Model 12 16 gauge 28" full choke with #7-1/2 when home on leave. The #7-1/2 loads were his choice for years in 16 gauge because they contained more shot pellets than #6. After he tired of shucking the Model 12 he got a Ruger Red Label O/U 12 gauge (mod/IC chokes) shooting #6 shot and was happy as a clam, except he could rarely hit anything with it (chuckle). I think he liked that it was shorter than the Model 12 and a bit lighter to carry, and always had the excuse that an O/U only had two shots and that was all he could get off at one bird.
With the 16 I hit a rooster at a paced off 65 yards using lead in 1972. Longest shot I ever made, and even though I worked at a trap/skeet shooting park ('69-'70 Roberts' Dairy Farm, Elkhorn NE: it no longer exists and is now the site of a community college. Imagine that!) it was a lucky shot, even if it was a station 4 low-house shot at that larger distance. The bird was badly shot up (feathers flew) and the farmer whose land we were hunting upon said there was too much shot to pick out and he was glad it was MY bird. Chuckle!
Back to the OP. From what I have read years ago, bismuth is the best route if non-lead because it is fairly heavy and will probably penetrate the bird better, but I have no experience with it or steel. If the other posters are using #2 steel in lieu of #6 lead, that says volumes.
I am sorry this post is probably no help for you.
I enjoyed your post and the responses. Thank you!
Jim