The finished cannon with and without  flash to see the color better

The parts are mostly PVC pipe, some fittings a sprinkler valve, a pressure gauge (optional), an air valve and some glues

Fitted all the parts together before gluing

the sprinkler valve was the starting point. The air reservoir and barrel simply extended from it

Next step was  to finish the reservoir. Drilled holes in the cap to accept a pressure gauge and an air  valve to 'reload' the cannon and be safe doing it.

Cut the pipes to length, added the  pressure valve and made sure to cut the barrel longer than the valve

Used hose clamps to  hold the parts together while the glue dried

Glued a length of  PVC to hold the batteries and trigger (switch)

Simply wired three 9 volt batteries in series to achieve  27 volts I think the sprinkler valve wanted 24 so it worked just fine for the life of the cannon

Once mocked up and tested, the connections were  all soldered to  ensure operation under any conditions. Wrapped the exposed wires in plastic cable wrap  for looks and protection