Lead, Bismuth, Steel, modern shot is going to be encases in a plastic wad when it goes through the choke. The main problem was some old steel shot shells would rust together and leave as a solid wad and with the short ramped chokes on older guns that was a huge problem I don't think many of us will face today.
OP wants a full choke for fowl hunting and the only way he will know what works best for his gun is to try it. He could end up with Super full or IM as his optimum choke for the load he wants to use, nobody knows but his gun. Steel hardness is a moot point.
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