For the Browning Light Twelve-gget a dedicated slug barrel. Either a rifle sighted smooth bore or (best bet) rifled barrel with a cantilevered scope mount with a 2X or 4X SHOTGUN scope.
The Light Twelve is really a top notch (Cadillac/Mercedes) class of gun so don't mess with the nice shot barrel it came with.
When you get a barrel get the best you can afford and try to stick with Browning! Remember! Don't put dimestore tires on your nice Cadillac or it will ride like a dimestore car.

Now...."Go Get Ya One!"