This started as one of my sketch sketches like some of the previous images. You can see the struts and base are still very loose and not really defined.
I imagine its a remote controlled auto turret, a cross between something from Aliens and Metal Gear Solid.
I also imagine the small cylinder in front of the ammo cart is a part of a shock absorbing system that uses the wait of the ammo to help stabilize the gun while shooting, thus it can shoot faster and more accurately.
Wacom Intous 3 with CS3
(this is from no game and is my own property)
Have a good day!