Disclaimer: The content of this page is for informative and entertainment use only. The reader is not encouraged in any way shape or form to commit to the actions described in the following text. By reading past this disclaimer, the reader releases the author of this document from all responsibilities related to damage or personal injury that may be incurred as a consequence of acting upon the knowledge, information or lack thereof contained in this document.

I am a cheap bastard by nature. Ask anyone that knows me personally and they will confirm it. After building the final version of the grape rifle and coughing up a fat wad of cash for the water sprinkler valve I looked at it and said to myself "Kalishnakov is going to roll in his grave." This was definately not a simple reliable and rugged design.Wires, batteries, an off the shelf solenoid valve that opens on a whim when it is given a good whack - this is not only expensive to manufacture it is potentially dangerous. God knows about the long term reliability when dealing with a mechanical system this complex. I immediately went to work on a scaled down version of the cannon that's shown here. It took quite a bit of coaxing to get the little 3/4 inch barreled cannon to work reliably but in the end I knew that it was going to be a winner. It required less than half of the amount of air pressure to launch a projectile with the same results as the water sprinkler valve. The cloud of condensed water vapor that poured from the end of the barrel justified the effort. Not to mention the cost difference between the $18.00 valve and the $3.00  (plus less than 10 minutes labor) piston.

1. Cut 3" and 4" pipes to length.
2. Drill holes in the cleanout cap that will accomodate the riser/ball valve and the tire valve insert. Position the holes so that a pipe wrench will be able to purchase a hold on the cap without disturbing the riser and tire valve. Clean and cement the tire valve. Screw the ball valve to the riser. Clean and cement the riser and ball valve so that the valve arm is 90 degrees from the center axis of the pipe when the valve is closed and won't contact the anything if accidentally rolled on the ground.
3. Prepare the piston by removing any sprues or flashing on the 3" endcap. Size the piston to the inside diameter if the endcap. Hold it up to the light to expose any portions that are out of round. Carefully sand down and polish uneven areas.

This is very critical if you do not have access to a lathe. If the piston becomes out of round from excessive sanding or filing then it won't fit well to the bore of the cleanout. If it doesn't fit properly then an effective gas seal will not be attained and the piston will not move inside the bore when air pressure is applied or removed. If this does occur then sand the piston down to a point where a short section of bicycle inner tube can be stretched around the piston to form a gas seal inside the bore. Apply grease to the rubber seal so that it won't get stuck in the bore. Trust me - I've been there and done that. It's an effective remedy but it entails a fair amount of labor. If you are priveledged and have a lathe, turn down the stock of your desire to fit the inside of the bore.

4. Remove the ridge in the 4" to 2" reducer so that the 2" pipe can pass through the reducer but yet still fit snug.
5. Tap the reducer down the 2" pipe starting at the exit side of the barrel. Place the rubber endcap on the receiver side of the 2" pipe. Do not cement the rubber endcap. The rubber endcap should be flush with the end of the 4" pipe on the receiver side. Keep in mind the seating depth of the 4" pipe into the reducer. Refer to the X-Ray for relative positions.
6. Mark the position of the reducer on the pipe and slide the reducer back towards the exit side of the 2" pipe slightly. Apply cleaning solution, allow it to dry and then apply the cement and move the reducer back into position referring to the mark previously made. Quickly clean and cement the 4" pipe into the reducer. Make sure that the 2" pipe is centered inside the 4" pipe before the cement dries. If not, place wedges around the pipe that will center it while drying.
7. Clean and cement 4" coupling to the 4" pipe.
8. Place the cleanout inside the coupling in a manner that the piston will be able to travel the distance from the threads of the endcap to the rubber endcap on the 2" pipe. Keep in mind that the longer the travel of the piston away from the barrel, the more air that will be allowed to enter the pipe when firing. 2 inches of piston travel should be sufficient. Mark this distance. Clean and cement the encap taking care not to get any cleaner or cement into the bore of the cleanout.
9. Allow sufficient time for all joints to cure. Be patient, premature pressurizing will weaken the joints and result in unwanted leaks.
10. Remove any rough spots or lettering from the piston head and rubber endcap using sandpaper. Finish up with higher grades of sandpaper then steel wool. These parts must mate perfectly for a good seal.
11. Apply liberal amounts of grease to the piston and insert it into the bore. Check for a good seal by blowing into the end of the barrel. (another good reason to let the cement dry. A lung full of cement vapor will give you brain damage and make your nuts shrivel)
12. Apply grease to the cleanout cap threads and screw it into the cleanout.
13. Test the cannon by pressurizing it at 10 psi stages and firing it without a projectile. DO NOT PRESSURIZE BEYOND 60 PSI. Troubleshoot any leaks by applying soapy water. The cleanout may leak slightly at pressures above 40 psi. Do not cement the cleanout cap. You will need to clean the grease/potato waste from the piston area from time to time.

Raw Theory (The pressure - I cain't take no more!)

LOAD IT:
The above illustration shows a cut-away of the business end of the cannon before pressurizing the air reservior. The projectile has already been placed in the barrel. For safety reasons the cannon should always be loaded prior to pressurizing so that no part of the users body will be mutilated in the event of an accidental discharge during loading.
 

FILL IT:
During initial phase of pressurizing - The air pressure applied to tire valve flows into chamber behind the piston and pushes it against the rubber encap. This seals the air reservior from the barrel. Air leaks slowly around the piston slowly into the main reservior.
 

SCRATCH YOURSELF NERVOUSLY WAITING FOR IT TO EXPLODE:
After pressurizing reservior. Pressure between the chamber behind the piston and the main reservior equalizes. This
high pressure holds the piston tightly against the endcap. Another way to look at it is that the relative low pressure in the barrel seems like a vacuum compared to the reservior. This vacuum sucks the piston up against the rubber seal.
 

ALL HELL BREAKS LOOSE:
The ball valve is opened allowing the chamber behind the piston to depressurize. This creates a vacuum relative to the high pressure contained in the main reservior that sucks the piston away from the rubber endcap. Simultaneously the high pressure in the main reservior pushes the piston back. This breaks the seal and allows the high pressure air to enter the barrel and drive the projectile to it's final destination.
 

I have only outlined the basic design of this this cannon. Many other improvements can be made upon this design to accessorize it and make it more hardy for field use. The first part I would suggest to improve is the ball valve. A shroud should be made to protect it from damage should it be dropped. Electronic firing can be added if cost is no object by employing a water sprinkler valve in line with the ball valve (ball valve used as a safety measure).The pistol grip from the grape cannon can be used if you wish to use it as a shoulder fired AT-4 training aid. You can also build a tripod with a traversing and elevation mechanism. Sights and scope mounts are a definite must. Don't forget a handy dandy serrated potato cutter at the end of the barrel.