Spudgun #1: The First Step

What is a spudgun?

Essentially, a spudgun is a homemade cannon, made from readily available materials. Like just about all firearms, its purpose is accelerate a projectile down the barrel and shoot it some distance.

The Spudfiles wiki has a pretty good writeup about the different types of spudguns: http://www.spudfiles.com/spud_wiki/index.php?title=What_is_a_spudgun%3F

Here, the focus is on pneumatic spudguns, since I only build pneumatics. Reason is simple: combustions need fuel, which means paying for every shot. A pneumatic can easily use a bicycle pump and muscle power, which doesn't cost money.

While obviously much less powerful than a real gun of equivalent size, a spudgun is much more versatile: pretty much anything can be a projectile, so long as it can go down the barrel.

Of course, like all guns, spudguns can be dangerous. Like real firearms, they can also explode, not to mention the obvious danger of someone getting shot. So, safety must be observed.

The First Step

Basically, all pneumatics can be split into 3 parts: the chamber to store compressed gas, the valve, and the barrel. Most of the improvements in spudguns are in the valve anyway.

This is my first spudgun, made in May 2008. It is extremely simple, utilizing a simple ball valve. While highly inefficient, the ball valve needs no modifications, is simple to assemble and is extremely reliable. It took 15 minutes to build and about S$10.

Here is a picture of the gun:

Most of the gun was constructed from PVC plumbing fittings. It is cheap, lightweight, and easy to obtain in various sizes, making it ideal. However, due to the high pressures involved here, only high pressure rated pipe can be used. It must also be solvent welded properly. Improper construction or insufficiently pressure rated pipe runs the risk of an explosion, which can injure or even kill.

The chamber is a used 500ml PET Coke bottle. The threads used for capping the bottle were filed off and the bottle neck epoxied into the 3/4” end of a 3/4”-1/2” PVC reducer fitting. A hole was drilled at the back of the bottle and a Schrader valve attached into the hole.

The Schrader valve was taken off a used bicycle tyre and the rubber removed from it. This is because epoxy does not bond to rubber, and it would thus leak with the rubber on.

There are no aiming sights fitted because ammo and firing pressure is never constant, and thus calibrating it is pointless.

Total cost: $15. Time spent building: $15.

Pressure testing (underwater) has shown that the Coke bottle chamber can withstand at least 160 psi, according to my bicycle pump gauge. Nevertheless, as PVC will degrade in sunlight, the gun must be stored out of sunlight.

Performance wise, it wasn't very efficient, but the sheer chamber volume gave it a lot of power: an AA battery shot at 120 psi at an angle of around 80 degrees can easily go over 5 storeys, as shown in the video.

The Next Step: PVC chambers

The first spudgun, though powerful, had one problem: the bottle wasn’t specifically designed for such high pressures and I had no idea how long it could last at the high pressures that I was using at. Not worth the risk anyway.

Hence, the next step was to simply switch PVC pipe instead of a Coke bottle for a chamber.

2 such guns were made: 1 with a chamber about the same volume as the coke bottle, and another with a very small chamber.

The first one had little difference from the original, while the underpowered gun was used for teaching purposes: with such low power, there was little risk of damage if someone used it wrongly.

The coke bottle ones were then decommissioned and given to Mr Jason Chan as a teaching aid for teaching the general gas law. It essentially allows students to see for themselves what happens at high pressures.