This thread talks a bit about cannon design and function https://rcwarshipcombat.com/threads/anatomy-of-a-fast-gun-cannon.408547/ Highly reccomend against building your own first set.
I can help you build your own (I have all the parts) or you can buy a Fletcher cannon from Battlers Connection for $35. The one I would help you build would be much lighter than the BC cannon. Besides the cannon you'll need a lightweight regulator and either a manual on/off CO2 valve or an electrically powered valve called a solenoid. http://battlersconnection.com/fletcher-50-round-cannon/
If you guys have a meet-up in late august/september I will happily come and use your help. Some stuff is a bit too complicated for an eighth grader that does software and not hardware
