One of the first things I noticed: The barrels are paper thin. To make them thicker extrude the region and click before it moves, then scale it down to how wide you want it. Repeat the first two steps but move it inward, so that it completes it's thickness.
Sandcrawler:
Sandcrawler:-Snip-
Is it a hover based tank, or you just havent added the wheels yet?
Should I add wheels or make it hover?