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 stage 0.
Using shoguns and repulses is useful against melee machines. Equipping a shotgun, lifter and any other ranged weapon on a surfer is a good biuld, is the lifter will lift the enemy up and the shotgun's volleys will push the backwards, possibly causing the enemy to flip.