Here's a gun I made for Steel Orbit Productions, Darx, The Crimson Guard, and the MOCpages Protection Agency.
It packs a punch, seriously. It can stand like a turret, and the bullets set on fire as soon as they are released from the mag,
but it's not automatic. Each time you make a shot, remove one of the treads (bullets) from the gun, which are on the side (third and last picture). Keep doing it until you run out, and open up the top and reload.
This whole process can take about an hour to complete a full round, counting the reloading.