All said after trading some guns, I have a new duck gun. Which I may or may not put Briley tubes in at some point, but over decoys with bismuth may work well enough.
You might be somewhat happy with the IC choke and modified chokes with bismuth, but you might find they're not quite enough to dishrag the big ducks. When I shoot bismuth I've got IM or tighter chokes. Wounded ducks are no fun, and they require extra shells to dispatch as they're swimming away ..............if you've got enough choke to do it. Sometimes several extra shells, and bismuth ain't cheap.
Steel would pattern much tighter in those chokes than bismuth. I shoot bismuth in my old vintage guns that I think really need it, but my 30" BSS gets fed a steady diet of steel. The 21 can handle steel every bit as well as the BSS, and you've got two great chokes there for steel over dekes, and even longer shots with the M (.020") barrel. JMO, YMMV.
SRH
