I decided to do this:
All shotguns in the book are loaded with Slugs (so those stats are balanced for range, accuracy, and damage).
Then, if you want to load birdshot or buckshot:
1. Halve the range increment (yes I realize what this does to the sawed off shotgun ... and thats what they get for increased hiding ability)
2. Add two more damage dice
3. For every range increment beyond the first, you subtract a damage die, and you get a range penalty like normal.
From my point of view ... this works ... it makes shotguns SUPER-Deadly at near point-blank ranges (up to 25ft), but then makes them not so deadly the farther you get away.
I might go through and specifically say certain ones (probably those with a 50ft range increment normally, and high damage 40ft range ones) only get ONE extra die of damage.
But that fixes shotguns in my mind!