Yeah - like that. Only instead of mechanical power, the force behind the bullet is from a very contained explosion. A little bit of gunpowder lit, then the energy of the blast expels it outward.

And yeah, exactly. Though it wouldn't kill everyone - that's why the decks are segmented. If one of them gets vented, the others'll still be sealed. If it's a small enough hole, it'll be a slow enough vent that you can fix it before you lose all your atmosphere, too.