Do you fire in the same manner or is it altered?
Is the damage the same damage for all creatures effected by the cone spread? ie 4d4 at 20th level?
does the feat just not work for any gun that only fires in a cone?
The blunderbuss has two options do I get to pick which type attack?
Spell Cartridges does specifically mention bullets, and says each bullet does xd4's damage. So perhaps its only for bullet based options?
The feat allows you to fire force bullets, not force pellets. They work like bullets--there is no cone spread. I don't see a reason why you couldn't fire the bullets from a normally scatter-only gun, though--the wording of the feat should get around that.
thanks