We built the a test version of a wheeled shooter this summer with Andymark 8" wheels, and VEX clamping gearboxes. See here:
https://goo.gl/photos/hjbNw8vYjDLF34nG9. After a bit of iteration (added a second stage, and reduced compression to only about 1") we got it shooting about 75 ft. There's still a bit of work to do before we can mount it on a robot (and I think we can tweak it to shoot further) but we might use the test version as-is as a "T-shirt mortar" into the stands at this fall's STEMley Cup