Newsgroups: rec.pyrotechnics
From: John De Armond
Subject: Re: Ze Potatoe (!) Gun
Date: Tue, 27 Jul 93 04:30:15 GMT

rats@cbnewsc.cb.att.com (Morris the Cat) writes:

>#The cops said children as young as 12 had been caught with these
>#things, and that they are illegal under the same laws that make
>#sawed-off shotguns illegal.
>#
>#The TV spot was carefully filmed to avoid revealing any detail how
>#to build one.  Could anyone provide a detailed set of instructions
>#how to build a monster potato gun?

>I don't know what the law has to say about such devices, but they are quite
>the rage back in my hometown.  I was there last week, and saw one that a 
>friend of mine built.  They are so popular that all the plumbing and 
>irrigation supply stores are sold out on the materials.

>My friend built his with a barrel length of about three feet, and a bore
>diameter of about two inches.  The combustion chamber was slightly larger
>in diameter, maybe five inches.  Standard plastic tubing adapters were
>used to connect the two different sized peices.  At the base of the barrel 
>was drilled a small hole, with a wooden dowel inserted and glued into place.
>This keeps the potato from being pushed too far with the ramrod.  The 
>muzzle end of the barrel was filed down to a sharp edge all the way around.
>This way, the user can take a potato larger than the bore diameter, and 
>manually force it into the muzzle, and the sharp edge trims it off to snugly
>fit in the bore, and a snug fit is necessary for good performance, just like
>a conventional gun.  At the very end of the gun was a threaded plastic fitting.
>This could be unscrewed to squirt a -small- amount of engine starting fluid,
>which is mostly ethyl ether.  

I have a similar design that is much more powerful and cannot have firearms
laws applied because the propellant is air.

The gun consists of a CPVC tube of appropriate diameter for the 
available ammo.  Mounted above that tube is a larger tube of at 
least the same volume as the barrel that serves as the air resevoir.
The resevoir is fitted with a shraeder valve for filling.
This tube is connected to the barrel by a couple of 2" elbows and a large
bore pilot valve.  I recommend wrapping the pressure tank with a couple
of zig-zag layers of filament packing tape.  Testing a tank to failure
shows the tape restrains the shrapnel.  Even plastic shrapnel can
be dangerous.  A small "purge" valve should also be fitted to the rear
end of the barrel so that it can be opened to release trapped air during
loading.

The valve is the key.  Home Depot sells a pilot operated water valve
for use on garden watering systems.  What pilot-operated means is the main
valve is operated by a diaphram.  Pressure is switched to this diaphram
via a small solenoid pilot valve.  The virtue of this valve is it opens
very rapidly when pressure is rapidly applied AND the plug retracts completely
out of the flow stream.  With the solenoid valve, the toggle action is 
fairly slow because the solenoid is low capacity.  The trick is to 
remove the solenoid and replace it with a large bore (3/8" or better")
toggle valve.  I use one meant for industrial pneumatic controls.
It mounts just like a trigger :-)

Use is very simple.  Open the purge valve.  Load with the spud of your
choice.  Fill the resevoir with 150-200 psi air pressure.  Aim, brace,
pull the trigger and be amazed.  The large bore tubing and valve ensure
full pressure is maintained on the spud-jectile the whole length of the barrel.

While I haven't made one of these for spuds, I HAVE made them for golf
balls and single-serving juice cans full of concrete.  It will blast a 
golf ball through a 1" thick piece of pine, splintering it all over
the place.  The juice can is even more powerful.  Better ballistic
coefficient :-)  The recoil is significant, particularly with the 
juice can.  Be prepared.

John