i dunno about that design, the co2 will either have to be dumped out or itll shoot out the bottom unless it has some kind of clamp or screw to hold it in place.
id just build the airsoft howitzer, much funner lol
http://m.youtube.com/watch?v=3yc3fAC5vOI
or at least build a mortar on te same basic design as the howitzer. If you google airsoft howitzer he has a crash course on how to make one so u can get some ideas there