Tungsten/polymer shot would also work well.
Myself, I would use steel with certain conditions:
The choke would have to be no tighter than Modified
Shot size no larger than #4 with Mod; #2 for IC
Velocity would have to be "normal" in the 1400 fps range or less
I glean this from personal experiences, the experiences of others, and the various writings and cautions of many different experimenters over the last 40 one years. I've seen only one barrel bulge of the many "unsuitable" barrels I and others have put steal down and that one was a Miroku O/U with Full choke. The barrel had a bulge one could feel much like the bulge in the early Browning Invector choke (and many other brands of that era) shotguns. The barrel solder was still firm and the gun is still going strong though the Full barrel has been opened to a loose Mod for further non-toxic use.
I've seen similar bulges from barrel obstructions after which the guns have performed as intended. The adage of "no steel" in older guns seems to be directed at the lowest common denominator shooter who seems to have problems determining just what gauge shotgun they have let alone understand anything regarding shot material, size, velocity, choke, et al. These types stop listening between, "yes you can" and "under these conditions..."
