Please tell me you're not using steel shot! Aside from doing irrepairable damage to the barrels/chokes, modern 2 3/4" steel waterfowling loads generate way more pressure than is prudent to put through any Smith gun, even the beefy Long Range guns. It's like feeding your old maiden aunt a diet of vodka and prime rib and then taking her out dancing all night, and not expecting her to get sick.
I won't even presume to imagine you would even think of stuffing 3" magnum steel loads into an old Smith gun. There are many alternatives one can employ to keep these fine guns in the waterfowling game.
The cause for your misses is indeed a mystery no one can diagnose from afar. I would first look to such things as gun fit, shooting style, and the fact that you may well be flinching from having shot all of those heavy loads in a relatively light gun.