The White Scars have the nonexistent Dreadnaught as a forbidden playbook - is there going to be one, or is that supposed to be Devastator?

I'd probably throw some wild west imagery in - cowboy boots/hats, sheriff's stars, etc - but that might not apply in your game.

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.