Yeah, the cannon was salvaged from one of my first airsoft gadgets, a recoiless rifle:

The design is stupidly simple compared to most of my new stuff but requires an annoying air pump/complicated CO2 setup. Its just a ball valve cannon with a schrader valve at the back. Hence why it rarely sees use.
Edited:
That is one of the baddest tank concepts I've ever seen, usually it's technicals, cardboard boxes with holes, or some other completely gay substitute. It's terrifying to look at, and it doesn't even have any mounted guns on it.
You should put some kind of "armor" plating (cardboard or something) on the sides, make gun ports, and have a raised seat and turret on top to serve as either a commander's seat, assuming it has more than one crewman, or as a SAW mount.
Its gonna be a tank, not some silly gun truck. Im thinking more along the lines of a wood frame with polycarb applique armor. You can go pretty far with these designs if you have the right tools/knowledge/number of crazy friends with technical backgrounds.
Case in point (Based off the same vehicle I posted, minus the tracks)
