Backpack Water Cannon - 8 Litre

Summer's here, and what better way too cool down than with 8L of water to spray at your friends!
I am going to take you through how to make a water cannon which holds 8 litres of water and never needs pumping!. It uses a bladdered accumulator tank and a custom firing assembly which can fire in 2 modes - riot blast and standard stream.
Prices and stores won't be quite so relevant outside the UK, and in America it may be more appropriate to use 'pressure rated' PVC pipe. Unlike in potato cannons you can use unrated pipe, because you don't need high pressure to get good results

Step 1 Materials

Most of my materials were sourced from BES, Toolstation, or Hendersons Plastic's.
A few parts did come from local stores, but I did end up needing the more varied range at BES. My local hardware store is pretty small, but you may be able to find the parts at a large plumbing store.
3m of 22mm copper pipe will cost around �10 but some places do sell shorter lengths. You will need about 1m depending on the size of your gun..
Endfeed is just a normal copper fitting. You can use compression fittings but these are uglier, more expensive and heavier.
The gun could be constructed to a simpler design with only 1 ballvalve and 2 tees, but this means no riot blast function
You'll need:

For the backpack

• 1x Expansion tank �18
• 1x 3/4" male x female full bore ball valve �6.50
• 1x Sturdy backpack �2 (charity shop)
• 1x 3/4" threaded coupler (used for filling the tank) �1

For the backpack/gun connection

• 1x 1.5m 19mm hose (female 3/4" threads at each end) �9
• 1x 22mm x 3/4" M compression couplers �2

For the Gun

• 1x 1m copper pipe �5
• 4x Endfeed tee joints �3.50
• 2x Endfeed endcaps �1
• 2x 22mm Compression Ball valves �7