Not a gun cart...but, if your looking to build one quick and dirty and have something fairly solid: Target has this wooden wagon for $23.10 that is ALL wood, with wood side rails, metal axles and pneumatic tires. Hell, the tires and axels alone are worth twice the price. It's going to be the chassis for my gun cart as I need to build one by Sunday.
Orchard supply sells a slightly nicer model for $119!
Bruce