So a few weeks ago I posted this thread about building a mini crossbow. http://grassrootsmotorsports.com/forum/off-topic-discussion/weekend-fun-mini-crossbows-for-the-kids/47171/page1/
After that I thought I’d build a bigger one, but then I watch ‘Troll hunter’ as referenced here
http://grassrootsmotorsports.com/forum/off-topic-discussion/great-film-troll-hunter/47874/page1/
and thought I better build something bigger to defend against trolls or other meanies.
So better living though ballistics was hatched.
Its spud gun time.
Normally spud guns have a 4” chamber and 2” barrel schedule 20 PVC pipe. All I had lying around was 2” and 3” pipe so I used that. Google indicates that you want a 1.5:1 chamber to barrel volume for max efficiency. Any less and you’re wasting the expansion of the propellant, any more and it’s fully expanded and you’re just adding friction in the barrel and slowing down the projectile. You need a 3” clean out plug and a 3” to 2” reducer. I already had some 3” – 2.5” and 2.5” to 2” reducers so I used those joined together to minimize what I needed to buy. The igniter is simply a pezo electric BBQ lighter. I extended the leads so I could have the igniter at the front of the barrel on the handle (an old electric drill handle I never use) so it’s close the CofG. It’s actually a little further back than that but not bad. I then used a couple of 2” #8 screws that I sharpened to a point with a bench grinder for the electrodes and used washers and nuts to bolt them through the chamber wall at 90deg separation. I put them at the back near the cleanout for a couple of reasons. First I wanted them accessible if I needed to service them or change the gap. Secondly I wanted them at the back of the chamber so ignition would start there and spread forward towards the spud, not back.
Here’s the parts laid out.
I sharpened the tip of the barrel with a die grinder and a coarse aluminum bit, and then finished with 80 grit. It’s now sharp enough to cut through a potato as you load it.
I’ve only fired it at home. Never again. First it sounds like a real rifle and one neighbor came running out to see what had happened. Second the spud went a long way and we never found it. This thing is potentially dangerous. This is not a kid’s toy. I’m heading to my in laws on Sunday so we can fire it off their deck, over the river and into woods safely. I’ll report back after that on it’s true potential.
Here are the electrodes.
Assembled
Master Troll Blitzer ready for action. 
ok.. that's just cool. Happy spudding!
This thread is worthless without pi^H^H vids.
Adrian_Thompson wrote: I sharpened the tip of the barrel with a die grinder and a coarse aluminum bit, and then finished with 80 grit. It’s now sharp enough to cut through a potato as you load it. I’ve only fired it at home. Never again. First it sounds like a real rifle and one neighbor came running out to see what had happened. Second the spud went a long way and we never found it. This thing is potentially dangerous. This is not a kid’s toy. I’m heading to my in laws on Sunday so we can fire it off their deck, over the river and into woods safely. I’ll report back after that on it’s true potential. Here are the electrodes.
Jeez. Couldn't you at least wear something black or maybe even a helmet? You look like you might be taking this troll thing a little too lightly.
My brothers and I did that several years ago but we used paper towels for wadding and small potatoes. We used hair spray as propellant. It made a helluva bang and the potato simply vanished, the paper towel shredded and bits of it were burning. 
Someone on this board mentioned firing one where they thought that no one was around, it turned out their spuds were landing inside the fence of a prison nearby!
Did one as a highschool physics project (Project Blackshaft, 6 feet long, still have it).
Insane fun and more power then you expect.
What are you using as propellant? I use either hairspray or propane in mine, but I prefer propane as the hairspray leaves a gooey mess after awhile.
For added fun, fire an onion at a solid surface. you just get pedals (or pulverized goo)
Secure chicken wire to a STURDY frame. Fire potato at it.
I used to shoot at a galvanized garbage can, it caved it in.
Fire it after dark, enjoy fireball out the barrel.
If your going to be firing it repeatedly, get a handhold fan to move some air through it between rounds. Without that I usually get 2 shots before performance really suffers.
Thought about hunting wild turkey with my potato gun at my moms house for instant turkey and mashed potatoes. 
Giant Purple Snorklewacker wrote:Adrian_Thompson wrote: Master Troll Blitzer ready for action.Jeez. Couldn't you at least wear something black or maybe even a helmet? You look like you might be taking this troll thing a little too lightly.
No worries, trolls don't come out during the day, I'll be ready by night fall
Curmudgeon wrote: My brothers and I did that several years ago but we used paper towels for wadding and small potatoes. We used hair spray as propellant. It made a helluva bang and the potato simply vanished, the paper towel shredded and bits of it were burning.
Yup. hair spray as a propellant is what I'm using. I'll try hte paper towell and small spuds some time.
Awesome story about the prison. that's great.
just curious... how much recoil is there?
Grtechguy wrote: just curious... how much recoil is there?
Essentialy zero on the one shot so far. But I was so surprised at the bang and the vanished potato I may not have noticed. Sunday after Easter dinner is the real test.
White PVC and spud guns don't mix, they will blowup after a good number of shots.
Just a FYI
Grtechguy wrote: just curious... how much recoil is there?
On mine there is a gentle push. You dont hold it tight to your shoulder, so its hard to compare with an actual firearm. I know I have rifles that are worse.
Many years ago, we used to battles with something similar. Back when soda/beer cans were tin, we used to cut the top and bottom off of several cans and tape them to make a tube leaving the tip and bottom on the bottom can. Poke several hole in the top of the bottom can before taping it to the tube and a couple holes in the bottom can. Fill the bottom can about half full of lighter fluid and slide a tennis ball into the tube. Aim, stand off to the side and put a lighter to a hole in the bottom can and it will go off like a cannon. We used to shoot them across to another barracks building when I was a youngster in the army. We called them tennis ball mortars. Sometimes the tennis ball would get lighter fluid on it and come out on fire.
I've built a number of these over the years and have slowly refined the design. These are the last two I've built. The black one is about 40" long and we didn't measure the second one but that's my brother and he's 5'7".
The short one has a nice deep sound to it while the long gun snaps like no other tater gun I've heard. Note the muzzle break on the long gun. It helps reduce tumble.
Next one is going to be a long version with a rifled barrel.
Edit> Forgot to note that those are double wall expansion chambers for extra safety.
When I was an early teen I somehow ended up bored and in possession of an ~3' long, very thick cardboard tube that had the perfect ID for a tennis ball. I had seen friends build tennis ball cannons out of tennis ball cans and thought "this should work just as well!" I plugged the bottom, knocked a hole in the side of the bottom, poured a few teaspoons of gasoline in, dropped the tennis ball in and held a lighter to the hole. Nothing happened! 
I decided that the reason it didn't work was because I needed the ball to sit slightly above the gas so I held the lighter to the hole and dropped the ball in. Well that didn't work but COOL-TINY FLAMETHROWER OUT OF THE IGNITION HOLE! I of course decided that I should put more gas in, light the ignition hole, and blow into the top of the tube to power my new flamethrower.................problem is the vacuum of the breath I took right before blowing into the tube full of gas vapors with a little fire at the end caused a fireball to travel up the tube and give me a nice dose of now very large flamethrower to the face. Burned off my eyebrows, crisped my wanabee teenager beard, and a roasted the ends of my nice southern mid 80's southern teen mullet! This thread makes me want to try again and do it right! 
wearymicrobe wrote: White PVC and spud guns don't mix, they will blowup after a good number of shots. Just a FYI
I've not heard an issue. How many times for those who have much experience with these things?
Are you thinking of the wack-a-loons who use propane instead of hairspray? That will make them pop.
OK, so spud gun testing was a massive success. Shooting off the balcony I got at least 300’, the trouble is I kept hitting trees so I don’t know what the full range would be. When fired vertically into the air estimates (of those who had not yet been sampling fermented grape juice) was 200-300’ as well. I guess I could weigh a ‘spud pellet’ and calculate muzzle velocity or energy from that estimates. Nice report, very little re-coil. I did have to be careful how I held it. As I extended the leads on the igniter with regular wire, not some kind of HT wire, if I was touching the wires I could feel a mild shock and it reduced the spark to the point where it couldn’t ignite the hair spray. All in all a lot of fun. The only problem was all the kids want one. Err no, this is not a kids toy, it’s a grownups toy.
Oh, and for all you ZAV fans you have to get a spud gun for back up armament. Once your regular ammo is gone, if you don’t want to go close quarters and hack them to pieces with a machete, instead you can pick up discarded body parts and fire them back at eh zombies with a spud gun, or would that be flesh gun?
Your next step should be a suppressor.
MG Bryan wrote: Your next step should be a suppressor.
Tell me more.
Next you need a length of steel pipe, and a proper propellent. A pound of black powder ought to do it - ofcourse - it will be a mashed potato shooter at that point so you need some bowling balls....
I always had good luck with a much smaller chamber, about 10 inches long. The best fuel that I ever used was 10% nitromethane, the stuff used in radio controlled airplanes. 2 squirts from a spray bottle was all that it took. Walnuts in the husk make a deadly round, they will pentrate 1/2 inch plywood at 10-15 paces. Always hold the cannon with the breech plug to your side. It can get cross threaded and blow back into your breadbasket. I have only seen that happen once, but it was dark and fermentation was involved.
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