Proto Tater-Tot Gun 1

My first pneumatic handgun is an 18-inch-long beast built from PVC plumbing, and powered by a bike pump. It has turned out to be more fun than you can beat with a stick.

When explaining this airgun to people, I usually tell them it has three settings:

• Poot (5-15 psi): Gently lobs objects, suitable for indoor use. It makes a sound which can only be described as "poot".
• Thud (20-35 psi): This setting would be very painful if fired at a person. It propels objects with enough force to damage things.
• Crack (40-60 psi): Not quite as dangerous as a real gun, this much pressure can still do a lot of damage. Objects make a distinct cracking sound when they hit at this velocity. 40psi was enough to put a hole in my garage door (oops). The gun also makes a crackling sound for a few seconds after firing; I'm not quite sure why yet.

There is also a fourth setting, "Foom". If I release the pressure very slowly, the gun makes a loud, long tone. It's neat.

If you build the gun well, you should also be able to try out the "BOOM" setting (70-120 psi). This can be dangerous. Make sure to test the gun's pressure tolerance with water first. (fill it with water, then put in more pressure than you intend to use; this is a fairly safe way to make sure it won't explode)

I took a bunch of pictures of my spudguns, which are shown below. Also, look at the diagrams on the airgun theory page.

As usual, click on the image for a bigger version.

	The proto1, holding about 26 psi.
	The proto1, empty and open. Some things to note: The bike pump's hook is handy for carrying the gun on a belt. The thin white rod is part of a plastic coat hanger, and works to help cram ammo down the barrel. The tan thing at the bottom is a bullet, a Ground Bloom firecracker with its innards replaced by a pencil, then wrapped in tape.
	Taken apart. You can see the diaphragm on the right. Also notice that the barrel really does go all the way to the back end of the gun.
	This is my new trigger. It's much easier to use than the ball valve, but slightly decreases the gun's power. It also tends to make people think the back of the gun is the front.

It will probably cost around $30 to build one of these guns. I paid more, but that's because I got some expensive parts for added effect.

Shopping list:

• A foot or so of 2" PVC pipe
• Two feet of 1/2" PVC pipe
• PVC size-adapter parts for 2" -> 1/2"
• A 2" pipe to inside thread connector
• PVC size-adapter parts for 2" -> 3/4", such that the largest piece has threads on the outside, and the small end has threads on the inside
• A very short 3/4" threaded PVC pipe (a.k.a. a "nipple")
• 3/4" Ball Valve, or... UPDATE! I've found that hose-squirting nozzles work quite well, too! (the kind which has a handle to squeeze) I'll try to get pictures soon...
• Screw-in air intake valve, similar to those on tires
• Bike pump with a hose, or an air compressor
• Stiff, flexible rubber sheet for a diaphragm. Many materials will work. I cut up a rubber 4" pipe cap.
• PVC pipe glue and cleaner
• Teflon tape for sealing threaded pipes
• PSI gauge which can do 0-100 PSI
• A drill, and a Tap and Die set would be helpful too

You'll have to search a bit for a good diaphragm, a good bike pump, a useable ball valve, and the parts for the two size adapters.

IMPORTANT: Make sure you have a lip inside the back of the 2" pipe or size adapter, because the diaphragm needs to rest against something! You'll probably want to get a screw-in size adapter so you can use it as the lip for the diaphragm.

Instructions:
Well... Just put everything together as shown in the pictures and diagrams. Don't glue on the size adapter for the back of the gun, because you'll need to open it occasionally to adjust or replace the diaphragm.

It may be worth noting that drilling and threading holes using the tap and die set can be difficult if you've never done it before. But you really do need to thread the holes, since the parts won't work otherwise.

(Better Instructions Soon?)

Being the nerd I am, I felt compelled to test this gun as accurately as I could; to figure out the physics involved... I first tried shooting straight up and recording the time until landing, but this gave me really inaccurate results. So, I went to a paintball store and used their velocity tester. Here's what it told me:

PSI	10 PSI	20 PSI	30 PSI	40 PSI	50 PSI	60 PSI
Velocity	106 ft/s	192	243	286	324	356

For comparison, an average handgun fires at 1000 feet per second, and rifles can fire at 3000 feet per second. So, this pneumatic gun isn't as deadly as a regular firearm, but you should still follow gun safety rules while using it!

Here are the results for the "vertical shot" test, using a tape bullet instead of a paintball. The estimated velocity from these measurements (based on simple Newtonion physics) are inaccurate by a factor of 4 or so, according to the paintball store's equipment.

PSI	Hang Time	Height	Velocity
12 psi	3.69 s	54 feet up	59 feet/s
20	4.56	83	73
30	4.80	92	76
40	5.43	117	86
50	6.06	146	97

The results for the vertical shots are very inaccurate. The bullet was severely affected by air resistance, which I didn't account for. Also, the timings have a margin of error of almost 1 full second.

I've heard so many diaphragm jokes since building this gun that they're not funny any more. :)

The diaphragm I'm using is the second one I've tried. The first was made from a plastic coffee can lid with part of a rubber jar-opener attached. It worked great for pressures up to 40 PSI, but got dislodged at higher pressures. The new one seems to have no upper pressure limit so far.

My gun does something weird if I release the pressure slowly. If I hold the release valve barely open, the gun will make a long, loud resonant tone. It's neat. The first diaphragm didn't do this, but it did make a goose-honking sound. :)

My diaphragm is stiff enough that it holds about 6 PSI after shooting. This leaks out over a minute or two.

The diaphragm also seems unable to hold pressures of 6-10 PSI for very long. There doesn't seem to be enough pressure to keep it from leaking out. However, my old diaphragm held this just fine.