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Pneumatic Spud Shooter

PURPOSE

Testing of hairspray spud guns revealed they can develop no more than 10-20 PSI chamber pressure. Most are built from schedule 40 PVC pipe. 3″ PVC pipe has a burst pressure of 260 PSI. People have suggested using other fuels to boost pressures higher, however this is much to dangerous. More volatile fuels can be very unpredictable and create pressures that would shatter the PVC pipe. Compressed air can be regulated to exact pressures making it an excellent propellant. The trick is releasing a large volume of air at once to launch the projectile. Here’s how it’s done…

DRAWINGS

You may need a drawing to fully understand the concepts of operation and assembly.

Pneumatic Spud Shooter

PARTS

All pipe and fittings are schedule 40 PVC

  • 1 1/2″ pipe 72″ long
  • 3″ pipe 36″ long
  • 1 1/2″ to 3″ bell reducer
  • 3″ female adapter
  • 3″ cleanout plug
  • diaphragm (see below)
  • a couple old trouser legs
  • source of compressed air
  • blow gun with tapered nozzle (What’s a blow gun? It’s a valve/nozzle combination that attaches to a compressed air hose. They’re commonly used in machine shops and garages to blow dust away. You can usually find one at most autoparts stores.)

ASSEMBLY

Cut the pipes to length. The barrel pipe must have a perfectly square and smooth surface on the breach end. You may have to flat file and sand it to make sure a good seal with the diaphragm is achieved.

Cut a 3 3/16 diameter disk of 3/32 thick polypropylene or polyethylene. A cheap dishpan from Kmart may be a good source of diaphragm material.

The front bell reducer has a small ridge that would prevent barrel from being pushed through it during assembly. Use a half round file to remove the ridge. Make sure you can push the barrel though the reducer. Remove the barrel and set it aside.

Drill a hole through the center of the cleanout plug. Size this hole to achieve a tight fit with the tapered nozzle of the blow gun.

Assemble the outer chamber first. Glue the bell reducer and the female adapter to each end of the 3″ pipe. allow the glue to dry before proceeding to install the barrel.

Place the diaphragm in the female adapter and screw the cleanout plug in firmly. Push the barrel through the reducer until it sets firmly against the diaphragm. Pull it back out 3 inches to allow glue to be applied. Apply glue to the barrel in front of the reducer and push it quickly back into position against the diaphragm. Hold the barrel in place until the glue dries.

After the gun is assembled, remove the cleanout plug and check to see if the barrel is properly centered in the female adapter at the breach. If it is not, cut 3 or 4 pieces of 5/8″ heater hose about 1″ long and force them between the barrel and the outer chamber.

Slip the old trouser legs over the chamber area of the cannon and tape them in place. This simple precaution will contain any pieces of exploding pipe from becoming shrapnel if something should go wrong.

FIRING

To fire the cannon, unscrew the cleanout plug and remove the diaphragm. Push a pre-cut spud into the breach of the barrel, making sure it is an inch or two below flush. Replace the diaphragm and screw the cleanout plug in firmly. Point the cannon down range. Place the blowgun in the charging hole drilled centered in the cleanout plug. Start squirting air into the cannon with the blowgun. When the sound of hissing air quiets down or stops, the cannon is charged and ready to fire. Keep the blowgun valve pressed down. Aim carefully and pull the blowgun from the charging hole.

TIPS

A tapered nozzle on the blowgun is a must. Size the hole in the cleanout plug to fit the nozzle snugly. Sharpen a short piece of 1 1/2 pipe on the outside to precut your potato. Portability can be had with a 5 or 10 Lb. bottle of CO2 and a regulator. feel free to experiment with different barrel and chamber lengths. This plan was only a starting point. Get a couple of friends to help spot your shots. The spud leaves the barrel *REAL* fast and can be hard to see. I’ve got one shot to go 755 yards!

THEORY

Because the diaphragm is somewhat flexible it will bend it’s outer edges forward from the pressure and allow the air to fill the outer chamber. At the same time this forward pressure is sealing off the end of the barrel preventing air from escaping up the barrel. Pulling the blowgun out of the charging hole allows the air behind the diaphragm to escape to the atmosphere. At this point all the pressure stored in the outer chamber bends the diaphragm in the opposite direction sealing off on the outer edge of the cleanout plug. The diaphragm continues to bend under the pressure and unseals the barrel end allowing the stored air to escape up the barrel behind the spud. This in turn, propels the potato distances before unknown to mortal spud marksmen.

In the pneumatics industry, this device is called a quick exhaust valve. It’s commonly used on truck air brakes. In an emergency, the quick exhaust valve dumps air from the spring brake chambers to apply the brakes fast.

SAFETY

DO NOT FIRE THE CANNON AT ANYONE OR ANYTHING. At nearly 500 feet per second muzzle velocity, a potato would be lethal if it were to hit someone.

The pipe should be clearly marked SCH-40 260 PSI. ACCEPT NO SUBSTITUTIONS or you will be reinventing the pneumatic hand grenade.

When assembling the gun BE SURE TO USE THE PROPER TYPE OF PIPE CEMENT! Do not use cement made for ABS pipe. If you have any doubts about your gluing technique, GET PLUMBER TO HELP YOU!

USE ONLY 100 PSI MAX! This gives the plastic pipe a safety factor of more than 2.

KEEP THE CANNON AT ROOM TEMPERATURE! Exposure to cold temperatures can cause the pipe to become dangerously brittle.

As pipe ages it may be a good idea to build a new cannon and discard your old one. All plastics have a limited life span and it would be very difficult to estimate the safe life of the cannon. ALWAYS KEEP THE TROUSER LEGS IN PLACE ON THE CHAMBER WHEN FIRING!

BE CAREFUL and have fun!

Potato Bazooka Plans

You may want a couple of buddies to go in buying the material with you, or make 3 at the same time, as the pipe comes in 10 foot chunks. I bought all material at a local “Home Depot” for about $15.

Note on choice of material: I have seen and heard of plans for spud guns using PVC pipe. In fact, an article in the February issue of “Modern Gun” uses PVC. I chose to use schedule 40 ABS plastic. The black pipe usually used for sewerage. If you want to know why I chose ABS, take a chunk of PVC pipe. Hit it with a 25 lb sledge hammer. It fragments into many *sharp* pieces. Try this with ABS. The sledge hammer bounces off the pipe and smashes into your foot. But it didn’t break! (The pipe, that is, I don’t know about your foot). PVC also gets brittle with exposure to sunlight. ABS just gets hot. Only ever use schedule 40! That’s the thick stuff. It costs a little more, but not that much more. The bill of material says 10 foot lengths, only because that’s as small a piece as is normally sold.

Web master’s note: There does seem to be some confusion about what pipe is safest to use. Finding pressure rated ABS pipe is nearly impossible in many areas. Most spud shooters are therefore constructed of PVC pipe. If you stick to conventional propellants and do not use oxidizers, PVC should preform with adequate safety.

Bill of material

  • 1 10 foot piece of 2 inch diameter schedule 40 ABS pipe
  • 1 10 foot piece of 3 inch diameter schedule 40 ABS pipe
  • 1 3 inch to 2 inch reducing bushing
  • 1 3 inch coupling
  • 1 3 inch threaded (one side) coupling
  • 1 3 inch threaded end-cap
  • 1 can ABS solvent-weld pipe glue. NEVER USE PVC GLUE on ABS!!!
  • 1 Coleman sparker – these are easily found in any sporting goods store that has a decent camping section. They are normally made for putting inside a Coleman lantern or stove so you don’t need matches.

Construction

Step 1 – Cut the combustion chamber to size. Cut off a 14 inch section of the 3 inch diameter pipe. You don’t need the rest of the 10 foot length, so save it for future bazookas, or make one with a couple of buddies splitting the cost.

Step 2 – glue the 3inch to 2 inch bushing into one side of the 3 inch coupling, glue the other side of the coupling to one end of the 14 inch combustion chamber. Make sure the joints are clean first and be liberal with the glue.

Step 3 – glue the threaded coupling to the other end of the combustion chamber (using the slip-joint side, obviously) make sure the glue doesn’t run into the threads.

Step 4 – Cut the “barrel” to size. Cut off a 36 inch (3 foot) length of the 2 inch pipe. Glue this into the other end of the bushing you’ve glued to the combustion chamber. You should now have the complete gun, but it’s not ready for firing just yet.

Step 5 – Using a file, taper the “muzzle” for the last half an inch on the outside. This will serve to cut the potato as it’s rammed in.

Step 6 – You’ll need to mount the sparker inside the end cap. If you got the Coleman one, it is threaded and has two nuts with it. There is also an angled piece of metal meant to hold the ignitor inside a lantern. Take the knurled knob off the end of the shaft. Be careful – there’s an extra flint inside the knob. Unscrew the nut and discard the angled bit of metal. Drill a hole dead center in the ABS end cap of a diameter to take the shaft of the ignitor. Mount the ignitor inside the end cap, put the nut on the outside of the shaft and tighten until the ignitor is held in place. The shaft will slide back and forth, but won’t come out. Put the end knob back on and tighten the lock screw.

Step 7 – Make sure the glue has “cured”. I left mine overnight before firing.

Step 8 – make a ram rod. I used surplus 1/2 inch PVC pipe, 4 feet in length. A broom handle, etc. will do. Measure and make a mark about 2 feet 8 inches down the ram rod.

To fire: remove end cap. Ram a potato from the muzzle end. The tapered end will cut the potato to size. Make sure it has a good seal as you ram it down with the ramrod. Ram to the mark you made. I’ve found most misfires happen when there are gaps between the potato and the barrel where gasses can escape. Spray 2 – 5 seconds worth of cheap hair spray (white rain, aqua net) I’d use an “unscented” one if you can, or the gun stinks after a few shots! Start at 2 seconds and build up! After the hair spray, quickly screw in the end cap. One twist of the ignitor knob sends the spud skyward!

Safety

Once you shoot this, you’ll see the potato comes out with enough force, you wouldn’t want to be on the wrong side! Usual safety about pointing the muzzle etc. still apply. This is for fun only. I don’t make any guarantee you won’t blow your arse off. (You may laugh it off, however). Personally, I’d never use acetylene, starting fluid (ether), black powder, lighter fluid, gasoline etc. as a propellant, but you may not value your body parts as much .

You can get 3 shots off a big spud. Partially baked ones are fun – they seal in better and shoot farther, but they do break up and the barrel is a mess to clean up afterwards.

Clean up

Soap and water. Push a small towel through (here’s a case where it’s OK to clean from the muzzle). I’ve been shooting mine since 12/94 and have been having a barrel of laughs. The spuds will go nearly 200 yards! I plan to make the “220 swift” variety by coupling a one inch barrel to the three inch combustion chamber. I wonder if you put the barrels on threaded couplings you could have interchangeable barrels. Sort of an “Idaho Contender”.

Other tricks

Ram in a cardboard container from McDonalds from an order of large fries. Leave the fries in the container. The cardboard serves as a sabot and voila – a shotgun!

Warning:

These things have a tendency to attract every 8-12 year old kid in the neighborhood.


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