First post......so be gentle!
I have been playing with various forms of pneumatic cannons relating to paintball for some time.
Some of the design criteria for this particular build are:
Be portable
Look like artillery
Breech loading
COTS parts (commercial off the shelf)
Deliver 200 paintballs
Here are pics, still need to fab a front "shield" to look more authentic but used it asis very effectively in a Scenario game already:



Gets between 40 and 50 shots from a 20 lb. CO2 tank - depends on charge pressure as I vary between 60 and 100 PSI for range difference.
Shoots 200 paintballs a shot in a sabot. Sabot falls away in less than 25 feet and lets the balls fly alone.
Due to the SUPERIOR aerodynamic quality of paintballs...........yeah right!
Just shoot them up and let them fall gets 50 ish foot groups at 145 yards.
Use a pair of 3/4" aluminum QEVs from McMaster Carr for fire control with pushbutton bleed valves to fire mounted on the "console."
The QEVs are the weak link in the system as they are only rated for 130 PSI.
I have an inline regulator and a safety pop off valve set at 100 PSI to stay within the safety envelope.
Aiming can be challenging when the wind gets up, but shooting down wind in 15 MPH winds adds 20 to 30 yards but spreads the group somewhat.











