Spud Gun, Spudzooka, Potato Launcher, Combustion, Pneumatic...
It is important to remember that these 'spud guns' are not toys and demand the same cautions and responsibilities as traditional firearms.
Adult supervision during construction and operation is the number one safety
If you are under 18, get a smart adult to help you.
The second critical safety requirement is education. Using substandard materials or improper construction can result in serious injury or death. There now exists a wealth of information on the internet concerning spud gun construction. It is your responsibility educate yourself before building or operating any type launcher.
This launcher is unsafe. Do you know why? Click for answers.
Presented here are a few tips to get you started in the right direction.
Pipe selection: Only use schedule 40 or 80 PVC pipe that is labeled as pressure rated. Pressure ratings are stamped along the length of select pipe. You need at least one, if not all three of these markings. If it's not there, don't use it.
NSF-pw - 'pw' means rated for water pressure. Most fittings carry this mark only.
A few types of plastic pipe to avoid are any and all ABS or styrene pipe. Avoid the markings: 'cellular core' or ASTM 02241. DWV is a designation shared by many types of pipe and should be ignored.
With your new investments in PVC secured, consideration should be given to the launchers design. Of importance is the volume of the combustion chamber vs the volume of the barrel (C:B ratio). If the volume of the chamber and the barrel are the same, it is expressed as a 1:1 ratio. If the chamber is one and a half times the volume of the barrel, it is expressed as 1.5:1. An efficient and powerful C:B ratio is 1.5:1. Higher C:B ratios can be achieved but seldom produce more velocity as the potato has already left the barrel before the full force of the explosion is reached.
A high C:B ratio does not necessarily make the launcher unsafe. A launcher with a 4"x11" chamber fitted with a 1.5"x46" barrel would produce a 1.5:1 C:B ratio. If the barrel was cut down to 12 inches the new C:B ratio would be 5.8:1. This high ratio would operate safely but perform poorly. To build a safe launcher, it is important to consider the chamber's volume vs the barrel's diameter. While there have been no formal safety standards set, there are a few common sense guidelines. One rule of thumb is to use a barrel diameter no more than 1.5 times smaller than the chamber's. By this rule, a 4"diameter chamber could use a barrel no smaller than 1.5"diameter. There are many exceptions to this rule as it does not include the volume of the chamber. It is used only as a safety guideline.
After designing the launcher, it's time to cut the pipe to length and glue it all together. Use a medium bodied, clear PVC cement. Follow the directions printed on the can, they are accurate. Yes, you need to use primer. Don't forget, you're building a combustion cannon here. PVC primer is usually purple but also available in clear. PVC primer is pure solvent which softens the pipe while PVC cement is solvent mixed with a filler for sealing gaps in the connection. Follow the directions on the can of primer, also accurate. If you have already hooked up the ignition spark, now is NOT the time to test it. PVC cement fumes are highly flammable and can ignite at the worst times. Also, never leave a spud or other projectile unattended in the barrel. Especially with fumes in the chamber!
A few web sites offer plans to build a launcher with a BBQ igniter placed on the rear of the gun, in the center of the threaded cleanout cap. This is a dangerous design as launchers are routinely stood up on the rear cap during loading. This could easily result in a potato in the face, at 400 MPH! If there was a potato and the pushrod in the barrel, you might just get a 'javelin' driven through your head or neck. Either would most likely kill you...or worse. Always mount the trigger in a safe location and include an in-line safety switch.
Last stop, common sense.
Be Safe! Have Fun!