Th shotgun was realeased with Minigun , First Aid Kit and Lifter . It shoots an array of volleys that does damage depending how much volleys hits the enemy. Volleys push the enemy back, and each time the shotgun shoots, your machine will be pushed back as well.
To be able to obtain shotguns normally, you'll need to reach prestige 2 (You can get them in gang box sometimes without prestige)
Using shoguns and repulses is useful against melee machines. With a Whale or Surfer, equip a shotgun, lifter and any other ranged weapon on a surfer is a good build, because the lifter will lift the enemy up and the shotgun's volleys will push them backwards, possibly causing him to flip.