Planning duck hunting trip this November and currently own couple Win. Model 12's/12gauge and have no idea as to how many rounds will be shot during the hunt, but can you fire steel shot in the Mod. 12(limited useage) and not do harm to the muzzle/choke?? If it does do harm, that just gives me an excuse to buy another shotugun!! All feedback appreciated.

Steel shot doesn't deform as easily as lead and the shot cup is thicker.That means the pattern is smaller compared to lead.My Benelli gets Extra Extra Full pattern with steel BB and MOD choke !! Makers usually don't recommend tighter than MOD for steel .In any case you must always pattern your load as there are so many different loads with different shot etc.

Might have the barrel fitted for tubes, probably work ok with a moderate choke. Possible option, contact Browning and see if a barrel from one of the repro. M12's would fit the older gun. I am not sure but would guess they were ok with steel. I did this for an old Browning Auto-5 3" 12 ga.
Note, the really old M12's had (I believe) a nickle steel bbl. which would be a bit soft for steel.
C.G.B.