Kotori87... Gascan? These guys have experimented greatly with cannons and could give you a real good idea. My 7/32" quadruple cannon are probably similar and are 5.5-6ci Das Bütts
Generally about 2 cubic inches per barrel. Of course, it also is more art than science, but that is always the point I start from. Less volume will require higher pressure, more volume reduces the pressure, up to a point. I prefer to err on the side of more volume, since it's better to waste a bit of gas and be sure your cannon has enough air to operate reliably and reach maximum power. Nothing is worse than rebuilding a new cannon because it doesn't have enough power behind it.
