Shotguns firing solid slug ammo use their middle damage rating and suffer a -1 to hit at all ranges (no rifling). Thier range increment multiplies by 3.
So the TL-5 Shotgun in the book will do 2d6 damage, at -1 to hit, with a 9m range incrememnt.
A lot of stuff like this is covered in Traveller's Aide 1: Personal Weapons. Its worth picking up, especialyl at $5.
Shane
