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scared out of my mind

A place to ask general spud cannon related questions.
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scared out of my mind

Unread postAuthor: facist7 » Sun Mar 29, 2009 9:53 pm

i just built my first spud gun, and for a test i dry fired it with green gas (aka propane) and it was extremely loud and had a nice kick on it.

i am now afraid that if i put something in the gun it will explode...should i even try launching something or am i just paranoid, what should i do?!?!?!
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Unread postAuthor: suburban spudgunner » Sun Mar 29, 2009 10:01 pm

Well, in order to tell if your cannon is a potential bomb, we're going to need some more information (i.e. pipe rating, material, fittings, etc.)

Some pictures wouldn't hurt either.
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Unread postAuthor: Daltonultra » Sun Mar 29, 2009 10:02 pm

If you're that terribly paranoid, grab some wire and build some long leads for your ignition switch. Clamp it or strap it to something heavy and fire it remotely a few times from cover.
A long time ago, I tried a combustion using an ignition coil and sparkplug.
It was big enough to worry me, so I used my Dad's Kubota tractor as cover while using the battery as a power source...of course, I never did get the stupid thing to work, but still...
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Unread postAuthor: inonickname » Sun Mar 29, 2009 10:05 pm

Sorry, but have a quick think before you post. Tell us the concentration of fuel you're using, size of combustion chamber, materials constructed from and ratings and method of joining and any other factors and we can give you a closer answer.
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Unread postAuthor: facist7 » Sun Mar 29, 2009 10:09 pm

the piping is sch. 40 abs, the chamber is 20 inches lond & 4 inchs in diamiter. the barrle is 4 feet long & 2 inches in diamiter.the connection between the 4-2 inch peices is a single peice.the whole cannon is glued together with primer and abs cement, and has set for 24 hours. i use a bbq ignition with a single spark point. and it looks like the most basic spud gun people make. i sprayed about 4 sec. of spray into the chamber.
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Unread postAuthor: inonickname » Sun Mar 29, 2009 10:18 pm

No, you don't understand. For a start, you haven't told us the diameter of the chamber or the barrel. Also, ABS is harder to find in pressure rated form than PVC. Last of all, Sch.40 denotes a wall thickness, NOT A PRESSURE RATING. After all, Sch.40 of foam won't do much will it.

Go and check that it has a pressure rating. If it says cellular core throw it out! Are you using threads, or was it solvent welded? If you can't find a pressure rating you should leave it or remote test it until you feel safe using it.
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Unread postAuthor: starman » Sun Mar 29, 2009 11:02 pm

First off, the term "green gas" is a made up, BS name...it's propane, pure and simple. Buy it in the welding section of your hardware store instead of the paintball store.... you'll pay a lot less for it there.

Combustion cannons are loud, especially without a projectile. You will find the cannon to be noticably less loud when you load that spud in there and fire it. Right now, the energy in your shot is being released as heat and noise only...it isn't having to perform any mechanical work.

As has been recommended in earlier posts, give us a few more details about your construction and cementing materials. We know you have a 4" chamber and 2" barrel. The only real no-nos are the use of cell core pipe for your chamber and the use of any cement other than pvc primer and cement. Other than that, it sounds like a basic spray and pray and you should be just fine.

If you could post a few clear, well lit photos, it would be helpful to us as well.
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Unread postAuthor: ralphd » Mon Mar 30, 2009 6:33 am

I can relate to your situation. I first built a spray-n-pray then modded to propane injection then modded a combustion fan then to tennis balls. The tennis balls fit so tightly that I was sure it would blow. I could almost choke on the amount of adrenaline that was flowing but it held again and again. I then modded to mapp gas and now it sounds like a shotgun and slams tennisball better than John McEnroe. As long as you used pressure rated pipe and fittings, glued everything well and don't double up on the propellant you should be fine.
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Unread postAuthor: Fnord » Mon Mar 30, 2009 10:07 am

The only thing I'm worried about is the primer. Did you use regular pvc primer, or was it made specifically for abs pipe? Don't use primer on a plastic it wasn't designed for, as it'll result in a weak joint.

Tons of people have used cell core abs without issues, including me. I can recall 2 instances of explosions, one where a wax slug sealed the barrel shut and one where an overtightened fitting caused stress on the chamber (sb15's gun).


You should be wearing eye/ear protection.
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Unread postAuthor: jwilson7 » Mon Mar 30, 2009 10:16 am

Two things... Safety Glasses, and a 1 inch piece of plywood...

At least if the gun does explode it might be entertaining...

And I agree with the others... MORE INFO!!!!(please?)
The pressure rating should be printed in grey on the side of your abs pipe.
Or at least thats where it is on some spare abs I have laying around the house.
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Unread postAuthor: jimmy101 » Mon Mar 30, 2009 10:33 am

From the description you should be fine. ABS, properly glued, shouldn't have any problems even if the round jams in the barrel, which is the worst case scenario for any combustion gun.

Make sure the fittings are fully seated. A common build error for a newbie is to only get a piece of pipe part way into the socket of the fitting. The pipe needs to be fully seated and glued for full strength.

The amount and type of fuel really doesn't matter, the energy (and maximum pressure) in the chamber is about the same for all fuels. The maximum pressure for a given fuel is obtained with a near stoichiometric mixture. If you put in too much fuel the maximum pressure drops. If you put in way too much fuel the gun won't fire at all.

Ralphd said If you "double up the propellant", by that he means doubling both the fuel and the oxidizer. That would be a hybrid and at 2X the peak chamber pressure would be pushing the pipes pressure rating if the round jammed in the barrel. If you just use double the amount of fuel (and the normal amount of air) then the peak pressure would be less than with a properly fueled gun.
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Unread postAuthor: jrrdw » Mon Mar 30, 2009 11:45 am

Sounds like your research amounted to coping off of the pictures here. You should read the post carefully to learn what you need to do instead of winging it.

In this hobby being lazy will get yourself hurt and it will be your own fault! I hope you take the time to read all the above post so the members didn't waste their time tring to help you.

I'm not giving you a hard time for no reason, it's for you own good.
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Unread postAuthor: SEAKING9006 » Mon Mar 30, 2009 12:50 pm

Also, check your state and local laws. I limit myself to pneumatics because combustions must be registered as firearms in Texas, though if you can avoid getting caught (read as 'have tons of land') you can get away with plenty of stuff. Appearing to be a college engineering student also helps.
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Unread postAuthor: facist7 » Mon Mar 30, 2009 8:59 pm

unfortunatly i am using cellular core, is there any way to make it stronger or do i absalutly have to throw it out and start from scratch?
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Unread postAuthor: daberno123 » Mon Mar 30, 2009 9:10 pm

Sorry, throw it out and start from scratch. A pressure rated PVC or ABS chamber is only a few bucks anyways. Think of as an investment in safety. :D
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