Upon reading descriptions of ”spud gun” construction from the net, specially Backyard Ballistics and The Ultimate Home Made Cannon page (RIP), I decided it was time to build my own. A bit like Don Quixote after too much reading, eh?

Alpha

Prototype stage.

Construction parts:

• 1 meter length of 75mm PVC pipe (I used Uponal 75x3.2), with one flanged one (”female”) and one straight end (”male”, fits inside ”female” end)
• 75mm plug for the flanged end of pipe
• panel-installable button-type piezo barbecue lighter
• 2mm screw, length 50mm with full threads

Firing:

• butane (I got a Ronson Multi-Fill® lighter refill can)
• wadding (I use a cheap fiber cloth kitchen wipe)
• ammo (whatever your sick little mind invents)

The pipe I used will give you a barrel inner diameter of 69mm which works very nicely with 67mm drink cans as ammo (both standard 0.33l or ”magnum” 0.5l cans work well). The piezo sparker can be a bit expensive and hardish to get, as BBQ isn’t as popular here as in the states. A good alternative would probably be a long-necked lighter. Attach it to the middle of the plug to fit the flanged end of the pipe. Wire it up so that you get a nice fat spark when you press the ”red button” on the outside of the plug. Insert the plug into the flanged end of the pipe, I haven’t even bothered to glue/tape it on at this phase since the inside flanging of the pipe has a rubber seal which makes the plug fit nice and tight. About 40cm from the straight end of the pipe (which will be the ”business end”) make a small hole and then screw the screw in it to set a lower limit for the ammo, preventing it from dropping thru to the firing chamber, ie. the lower half of the pipe. Make a small hole (I used a 2mm drill) a couple of cm below the screw, on the opposite side of the pipe for fuel injection.

Firing: stuff wadding and ammo thru top of pipe down till screw, spritz about 2 seconds of gaseous butane from can can using a small nozzle (you get a selection of adapter nozzles with refill cans) into the pipe thru the small side hole, wait 5 seconds for air and gas to properly mix (it won’t go anywhere, don’t worry), bonk pipe on ground to trigger piezo button. Thunk!

Debugging: in my first experiments I severely overdid the butane, and also held the can top-down as the instructions for lighter refill suggested. Semifluid butane in quantity ended up in the firing chamber. No good, the damn thing didn’t even fire as the rather slowly boiling butane replaced oxygen in the pipe. If you take care to put gaseous butane in (the can top upwards), the timing will be much more reliable.

Further experiments show that

• fuel injection timing is quite critical, it is very hard to get a good burn – most of the time you just get a little blurp and the ammo plops out of the pipe with belched flames
• getting the firing chamber aereated for the next shot is really hard; I haven’t seen any lightweight bottled oxygen available in Finland (only heavy-duty technical stuff), so a openable breech would seem to be the only possibility (not to mention safer!)

Beta

Gone thru several firings, though still considered experimental.

Additional parts:

• PVC expander with female 75mm and male 110mm connections
• PVC 150mm long ”female-female gender changer” assembly for 110mm pipe (sorry, I don’t know what these are really called)
• PVC cleanout assembly with screwable cap and male 110mm connection
• small plastic film canister to protect piezo sparker in firing chamber

Adding the expander assembly adds nicely to the capacity of the firing chamber, making fuel timing easier and adding to the lauch velocity (which also means a lot louder bangs). The 1 meter pipe is facing the other way now (with the old fuel injection hole taped over), so the ”drop-prevention” screw is about 60cm down from the ”business end” of the pipe. Fuel injection into the firing chamber is done thru a small hole on the expander. Getting oxygen into the chamber is a ”breeze” now, I’ve had no firing difficulties since adding the removable cleanout cap. Bang!

Rebuilt the piezo sparker mechanism onto the cleanout cap (I had to get the sparker replaced due to unrelated mechanical failure), placing it inside a small film canister, as it had seemed to get slightly frizzled by the hot gasses (and occasional soot) in the firing chamber.

A closeup view of the cleanout cap mechanism. On the left, the spark gap with the ground electrode wire visible, and the ”big red button” on the right.

The firing chamber pressure is a lot more with heavier payloads, actually so much that the expander arrangement can get blown clean off (I’ve still not attached it permanently to the main pipe, just using the rubber gasket system that this piping system comes with). Will have to work on it, maybe also adding some kind of insurance against catastrophical failure like an old pants leg to contain possible shrapnel. A professional I asked about availability of ABS parts said that this PVC system with rubber gasket sealing (normally not glued together) is about the only thing available around these parts. I’ve seen huge ABS parts though, but I’m not putting together Big Bertha.

Here is a picture of the beta phase device. The gun itself with the ”business end” pointing away from the camera is on the right, with the expander open and the cleanout cover (with the piezo sparker inside the film canister attached) sitting on the left. Some ammo cans (also with a 0.5 liter ”magnum” load) a bit worse for the wear can be seen by the pipe.

Gamma

Additional parts:

• glue suitable for gluing PVC parts, I used Bostik A3 contact glue
• old pants leg for protective sleeve
• metal hole strengtheners to put on holes in sleeve
• bit of leather for carrying strap
• one M8 bolt, two washers and two nuts

All piping is now glued together. This glue takes well to PVC, my guess is that you would have to crack the plastic parts to disconnect them from each other.

I’ve also sewn together (from an old pants leg, just as I planned) a sleeve around the gun to protect the user from possible catastrophic failure. Here is a view of the firing chamber seen from the front. The brown leather carrying strap and the new injection nozzle (a drilled-out M8 bolt) are visible. I deemed the nozzle necessary as adding fuel thru the small hole in the firing chamber was rather tricky (the amount/timing and also now finding the actual hole under the sleeve), and the small amount of backfire that came thru the fuel hole was making bad burn marks on the sleeve!

A view down the barrel from the firing chamber end. The cloth sleeve around the construction is clearly visible, as is the inner end of the injection nozzle (hexagonal bolt) and the screw a third of the way down the pipe. The injection nozzle is secured to a larger hole in the firing chamber using one washer and nut, and then the hole on the sleeve with the metal strengthener is placed over this and secured on with another washer and nut.

It was the real thing!

A can of unnamed soft drink after hitting a metal fire escape stairway.

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