Knights of Fate, Warrior Source book. Specifically the thunderhead description on pg 42. Near the end of the description it says something to the point of there being other shells available for purchase. Assuming the thunderhead functions like a standard shotgun of our own current era. the various ammo types would drastically alter the stats of the weapon. However I cannot seem to find a listing anywhere of what these bullets might do or cost Initial ideas, Flechette: gains either pierce or vicious Gel rounds: damage type becomes stun Slug: lose blast, increase 1 range band, and pierce of some level.