44 AMP you spoke volumes, however as a student of military and actual used guns, I have to disagree with you and others, that they arent good combat guns.
I certanly didn't mean to imply that they were not good combat shotguns. They are. What they aren't is a gun designed as a military weapon, purpose built for easy takedown, or fitted with all the "tactical" gadgets and latest innovations. One doesn't ususally see ghost ring sights or extended mag tubes or pistol grips on Win 12s, & 97s. NO need, really.
As to steel shot? I wouldn't. I've never used bismuth, it might be ok, but I hear they are pretty spendy shells. As for a replacement barrel, ain't gonna be cheap, and will take a gunsmith to install. They are threaded into the reciever extensions, and don't swap out like the ones on guns designed later. I wanted a riot length barrel for my model 12 for years (not willing to the the 30" full choke one cut), and wound up buying another model 12, one that somebody else had already cut down the barrel.
Another thing to remember is that these older shotguns, made before the advent of plastic shot cups in ammo, pattern tighter than what is standard for today. This is because in those days, using the paper shells, and fiber wads, card wads over the shot, no shot cups, choke was what ever inside diameter of barrel that would produce the desired percentage of pellets in the 30" circle @ 40yds.
My Grandfather's rule of thumb for telling if a gun was full choked was that you could balance a dime in the muzzle. You can do that with an old full choke Winchester. Try it with a new gun, and the dime will fall right through. Take a Winchester made before the late 1950s, and it will (generally) shoot one or more level tighter than the marked choke, with modern plastic shotshells. So a full choke is likely to shoot as extra full or super full. Myself, I wouldn't have the choke on an old model 12 or 97 altered in any way. I'd just learn to shoot it the way it was.
NO STEEL!!! Shooting steel though an old full choke gun could result in a bulged barrel, or maybe worse! You might get away with it for a while, or you might not. No sense taking the risk. Get a new Rem or Mossberg or something to shoot steel through.