The gunslinger penalty is pretty nasty - it knocks you to 3 average damage with hack-and-slash, behind the Cleric and Bard, and the Dedicated version gives you lower damage than the Wizard. I understand wanting lower damage for hack and slash than for volley, but that seems a bit excessive. I'd probably go with d6 damage and then add a line to the gun saying that your damage is d10 while using it, and leave the penalty off Gunslinger.
Armor-piercing rounds should indicate that it's optional and Da Bomb should note that you can't spend ammo if the volley comes up 7-9. The other interpretation would be dumb, but you should write it down anyway.