i killed a limit of teal saturday with my favorite m12. It's a 2 3/4" 28" mod with a simmons vent rib. The wood was beat up when I bought it, and it had been reblued. I hunt with it for almost all upland and early waterfowl trips. In fact I'm leaving here in the next few minutes to go dove hunting and I'm taking it with me. I've never messed with having the choke tubes installed although I've thought about it. The guys I know that have done it say that the results are better if done on a 3" "heavy duck" gun. Apparently the barrel thickness is important for the threading. That's all fine and good except that I can't think of a situation where I'd want to shoot a 30" 3" "heavy duck" gun at upland birds. I hate the poly-choke things with a passion and won't do that to my guns.
Chokes are over-rated anyway. I've said it before in other threads, but I'll say it here again. People who spend a bunch of time patterning different shells/chokes/guns would be better off to have taken that time and spent it in other ways. Practicing shooting/calling/scouting or even working would be more productive and put more birds on the table that worrying about your choke. I bought my work-horse duck gun (a 3" 26" BPS) many years ago and put the modified choke tube in. I haven't taken it out. I've shot fall turkeys, doves, quail, pheasants, ducks, geese, rails, snipe, squirrels, and even a few fur bearers since then. I've shot a perfect round of skeet and a perfect round of trap with that modified M12. Now don't over react to what I'm saying. My spring turkey gun has a super full choke in it. I also have a couple of benellis that i routinely change the choke on for my fiance. To say chokes make no difference would be false. What I'm saying is that they don't make THAT MUCH difference, and aren't really worth worrying about. 100% of the duck hunters I've met that worry about chokes have no idea what they're doing with a duck call, don't know how to work ducks, and can't hit what they're aiming at. Each of those things is more imporant than their choke/load/gun combination.
I've never shot bismuth. I've shot heavy shot, steel, lead, and copper plated lead out of that m12. I've never noticed any real difference. I even shot a 3" 30" Full M12 at ducks and geese using steel for part of a season without any negative results. I probably don't recommend doing that, but I did it.
My advice, buy a case of 2 3/4" steel 4s or 2s for ducks or lead 6s for pheasants and go hunt. Kill 'em all.