Interesting project, but the spud gun will probably be unreliable. A paintball gun might be a good replacement.
Best thing to use, if you need a "gun" on your robot, is an electric airsoft gun; very easy to interface with, and if you don't mind the cost, you can easily purchase steel and ceramic shot (ie, ball bearings) which will fit such guns (if you need something that will pack more punch than standard plastic airsoft bullets). I think they also make airsoft rounds that are essentially paint balls, as well as pepper spray...
Instead of converting an RC model tank, maybe a DIY wooden tracked setup like below might be the way to go.
What would be better than a wood tracked machine would be to use large double sided timing belts, with custom metal cleats attached to the outer edge (or just leave the rubber teeth as-is). A system like this was described by Martin Bradley Weinstein in his book "Android Design" (ISBN 0-8104-5192-1); note that this book is long out-of-print.
A simpler design, which offers performance close to that of tracks, is a six-wheel differential/skid-steering robot. Use of large 8-10 inch all-terrain pneumatic tire/wheels (like from Harbor Freight), a welded/bolted steel/aluminum box frame, and chain drives coupled to low-cost gear motors offers a very robust all-terrain platform. Such a platform has been described by various authors, and was also used by CMU for their Terragator platform:
