Here's one
McMaster-Carr,
http://www.mcmaster.com/#6498k485/=10n6o8z
That's one way to go, we are thinking of almost the same concept, and are preparing to arm many tanks on our bot this year.
Expensive, but it could work, honestly we are looking at putting 2 on the robot stored at an angle. Therefore, roughly half the power per cylinder is needed, but twice the air... the bore wouldn't need to be a mile wide though, which is a plus.