Sure looks like a gun to me. Depending on how it delivers the pepper, might legally meet the definition of one too. I see what might be a hammer, if it uses an explosive or primer to discharge a projectile (pepper ball) I think that makes it a firearm under WA state law.
ETA: Ah, here it is:
(7) "Firearm" means a weapon or device from which a projectile or projectiles may be fired by an explosive such as gunpowder.
Now, if that were a 12ga buckshot shell instead, that would make one messsy lil close-in BUG