Level 1: The scatter blaster is your classical single-fire shotgun. It has a slight delay between each shot to cycle the next shell. Exceptional weapon for killing Zombie Bots.
Level 2: Dual-barrel design, Consumes two shells per shot and fires two shells per shot. Double or nothing!
Level 3: Longer barrels, increased ammo capacity per load, the firing mechanism becomes a lead-pumping nightmare, enabling a rapid-fire more at the cost of a reload time after firing off every shell. (Easily the most overpowered weapon in the game!)