spud gun question ... UPDATED with pics

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I'd recommend using a compressed air cannon. Much more predictable, and much less sharp pressure spikes. Add in a foot-pedal bike pump, and you can get preciously close to the 100 psi sanity cutoff point. And you can get a ton more oomph, too.

My gun, the Dragonlance Mk.1 prototype (a former classmate has the first Mk.1 Production) is almost stupidly simple to build. Get five feet of 2" PVC, a 2" cleanout cap, and a 2" solvent weld ball valve. You'll need some PVC cement, and a Schrader valve from the auto parts store for filling. Cut the PVC into two pieces, one about 2 feet long to be the pressure chamber, and one about 3 feet long to be the barrel. Put the cleanest cut on the muzzle end of the barrel for looks. You'll want to drill a biggish hole in the short section of the pipe, about three inches from the end to be the stock, but use the smallest one you can get away with and still have the valve stem seal. Now comes the welding. Use lots of the purple primer, and don't be chinsey on cement. Remember, this is going to have to hold in a decent amount of energy. Take the center out of the cleanout plug, and cement it to the valve stem end of the chamber (or the other end; this is personal preference - I like mine out of the way). Then cement this to the ball valve, making sure the two line up; this'll help keep weight off of the valve if the gun is set on its side and rolls. Finally, cement in the barrel and let it sit for... oh, 24 hours is good, but 72 is better. Then, finally, check the valve for function (find one in the store that's a bit loose, most are really hard to open) and screw on the endcap with a bunch of teflon tape.

then air it up to increasing pressures, in 5 or 10 PSI increments, and launch convenient projectiles to get a feel for what it's capable of. My original uses Sch40 PVC, and I've used it at 100 psi a few times. Keep it out of the sun, and paint it -- I recommend a flamed barrel end, though this never got past some Sharpie on the prototype.

Another thing. Dixie cups are just the right size to make self-sealing sabots.
 
http://www.spudtech.com/detail.asp?id=32

Building a spud gun is some of the most fun you can have with kids. The glue needs to dry overnight.
Limes make the best fodder. I get 300 yards out of a well proportioned lime. You can also use a paring knife to perfect the potato. Cut a piece of pvc to check vegetable size at Freddy's. Wipe the moisture out of the ignition chamber to continue getting maximum effect.
 
Hi Pax, just a quick note to be CAREFUL with these things! Treat them as firearms.

A good spud gun can fire a spud in excess of THREE HUNDRED yards.

I've personally built one (of the conventional right-guard ignotion propelant style, albit with a frozen spud.) that was able to penetrate 3/4" OSB plywood (!) 5/8" drywall, exterior hardwood siding (long story, the shed was set for demolotion shortly :) and was last seen arcing up and away. We of course had a safe backstop in the mile or so of field behind the shed.

Be safe and HAVE FUN!
 
Chrontius said:
I'd recommend using a compressed air cannon. Much more predictable, and much less sharp pressure spikes. Add in a foot-pedal bike pump, and you can get preciously close to the 100 psi sanity cutoff point. And you can get a ton more oomph, too.
Ditto.

Did that last Christmas. I make my kids keep it under 75 PSI. Not really a problem - only the 12 year old can pump it any higher. :)

Oh, BTW, for you other homeschoolers, the mortar design with adjustable elevation is a good way to teach ballistics. ;)

Here's my crew:
 
Well, the scattered members of my family finally returned from various out-of-town locations, and so today we went out with the spud gun to make long-distance mashed potatoes.

On hand were me, husband Bob, sons Ben (16), Jeremy (14), David (12), Jonathan (11) and Timothy (10). Friends who couldn't bear to miss the excitement were buddies Don, Tom and Diane, and our friend Gila also stopped by for a little while. Don very graciously handled all tech support for the potato launcher and assisted with launchings, while Bob and I ran herd on the kids who weren't shooting.

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While waiting for Tom & Diane to arrive, the kids got a very active game of potato baseball going in the field. Youngest son Timothy was not happy however. "They're going to mash my potato!" he complained.

First shot honors went to Jonathan, who won the coin toss. Don showed him how to shove the potato down the barrel (henceforth, "down the tuber") with the masher, and then had him spritz hairspray into the chamber. A quick check around showed everyone wearing eye and ear protection, so Jonathan called, "Fire in the hole!" then hit the red button and launched the inaugural potato.

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pffffft!

The inaugural potato was a spud, landing only about 25 yards downrange. Not terrifically exciting, but Jonathan was pleased and the boys all cheered.

David won the next coin toss and loaded his own potato. "Can Ben go downrange and try to catch it, Mom?" Ben was willing, too. Kids ...! (Just in case there's any doubt, the answer was a resounding, "NO.")

PFFFFTTT!

"Oh yeah baby, that's what it's about," laughed buddy Tom. About twice as loud as the first launch, tater tot two also went over twice as far, landing beyond the hundred-yard mark. David stood up and hollered, "Yeeeeehaw!", giving me a high-five on his way past.

Several dozen potatoes later, I looked over to discover son Ben had carved himself a rifled potato slug. "Since we don't have a rifled barrel," he explained, "I think this should do it." Someone pointed out that the rifling might prevent a good gas seal. Ben immediately responded, "Lots of scrap lying around. I'll make a wad for it." And he did.

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That potato, surprisingly enough, did not break into french fries when launched. Instead it stayed nearly intact, and Ben was later able to retrieve it from its landing spot some 200 yards downrange.

Towards the end of the afternoon, after firing at least 25 pounds of taters, we had some difficulty with ignition. Despite cleaning the igniter ends and wiping out the chamber, we were unable to get the spudder to function reliably after that. Not sure what was wrong, but since the temp had dropped down into the 30's, we were all glad enough to head inside to warm up. We'll take it out again in a couple of days, and hopefully be able to diagnose the problem then.

It wasn't until we were on the way home that we realized we should not have been calling "fire in the hole!" upon pressing the switch. Obviously, the correct call would be, "Hot Potato!"

Many thanks again to all who helped us with links & suggestions for this...

pax

Boys will be boys, and so will a lot of middle-aged men. -- Kin Hubbard
 

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Sounds like a great Christmas for all involved.

I always had trouble with those piezo ignitors as well. I never got around to building it, but I had an idea for electric (battery) ignition: build what is, essentially, a taser. Capacitors and voltage doublers dumping their energy across a spark gap of a half-inch or so (using good, heavy bolts for the electrodes).

Give them something to experiment with, anyway.
 
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