Your choices are basically bismuth, NICE shot, tungsten matrix, or Hevishot classic doubles (not HEVISHOT in itself). If you reload, you could try Ballistic products ITX shot also.
I used steel shot in my 12 gauge 311, but it had more open (IC and Modified) chokes. My problem with steel shot in the smaller gauges is that the pellets are too big in order to kill ducks cleanly, so the patterns tend to be sparse.
