Here's a set of plans of the gun cart I built (as shown in the picture below). The cart was created in a few days of spare time with resources I had at hand. I've drawn up a set of plans and a how-to doc with pictures. The plans are free to anyone for personal use and not for commercial use or distribution. If you've got comments or questions, drop me an email at firstname.lastname@example.org.
Gun Cart Docs
If you don't have Adobe Acrobat installed to work with your brower, right-click on the links above and select "Save Target As...".