Helllloooo... lookee what I found: http://www.pukindogspaintball.com/cannons/BECC.htm
A breach-loading, high caliber, air cannon used for recrational paintball. Outfit that with a single solid foam round or a huge load of BBs and I think it would make a beutiful main gun for an airsoft tank don't you? You could screw with the design a bit to make resemble a tank cannon more, but the concept is good.
Make a good rocket launcher too