[asid61]

I like the form factor a lot, but I'm seconding Ollien's comment. Expanding it out by 3/4" or so would fix the problem.
The use of 1 wheel for 2 balls is very clever, but you might want to spring load the casters to keep contact with the wheels as they wear down. The issue of "drilling" into the ball with an un-moving wheel is still there, but maybe if you used thinner wheels and something like a metallic ball you might have better luck.

EDIT: What's the FPS and weight?