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Combustion bb gun?

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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Combustion bb gun?

Unread postAuthor: devilman » Tue Jan 29, 2008 7:01 pm

Im considering making a combustion powered bb gun. Im to sore and tired to pump my air one, and the long time between shots sucks.

I made a quick one out of some pipe fittings and a 100mm piece of pvc pipe, about 30cm long. I did a small test of it but i had some big leaks, because i only had a plug and a cleanout, so i had to cut a hole in the plug and screw in the metal pipe fittings.

I was wondering if i build it properly, what sort of power and noise would i get out of it, and also what could i use to join a pvc combustion chamber to 3/4" steel pipe fittings? Here is a quick sketch of what i did. I added the extra pipe insted of fitting the barrel to the chamber to try and decrese noise and power, and to add 2 places to hold it, one under the tee and the other at the right angel pipe fittings, where the spark trigger is mounted.

Suggestions welcome

*edit- if i rebuild it properly, it will look more like a rifle, so it is easyer to aim and fire, this was just out of bordem

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Unread postAuthor: bigbob12345 » Tue Jan 29, 2008 7:07 pm

You dont need such a huge chamber just the 3/4 in steel will be suffecient and you could make that pneumatic cause it would be so quick and easy to pump up.
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Unread postAuthor: devilman » Tue Jan 29, 2008 7:29 pm

i already had it as a pneumatic, i only have a bike pump and it killed getting the pressure up, also i got tired of piston valves. I wanted to make a combustion because its simple, and i dont get sore arms. I know i didnt need the huge chamber, it was all i had at the time :P

I carnt really use the 3/4' steel as a combustion chamber, i am useing hairspray as its cheap and easy to get and it condenses on the side of the pipe. Also i dont really feel like drilling out the steel and trying to install a bbq spark, then try not to get zapped :P

I want to use pvc as the chamber, as i have a endless free supply of it atm, and its easy to work with.
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Unread postAuthor: Novacastrian » Tue Jan 29, 2008 8:03 pm

Devilman- as said above a smaller chamber would suffice, i would recommend using a chamber fan if possible and also use metered fuel (syringe and can of Butane).
You can expect "decent power" from such a setup, probably like 450fps with a decent length barrel, that's enough power to put holes through aluminiun cans and such and like you said it saves a lot of time between shots compared to a bike pump :)
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Unread postAuthor: devilman » Tue Jan 29, 2008 8:54 pm

i have already decided that i am goign to use a chamber fan, as i have a ton of old pc bits hanging around. I have buntane, how would i use this mesured fuel thing? Im interested.

Im going to see what parts i can salvage this arvo, as i def need a smaller chamber. 50mm should work fine shouldent it? What length should my chamber be if my barrel is around 1.2m in length?
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Unread postAuthor: Eddbot » Tue Jan 29, 2008 10:56 pm

i'm also curious about the syringe method, i see alot of people talking about it but never understand what they're talking about
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Unread postAuthor: bigbob12345 » Tue Jan 29, 2008 11:21 pm

devilman wrote: What length should my chamber be if my barrel is around 1.2m in length?


as small as possible
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 11:30 pm

Eddbot wrote:i'm also curious about the syringe method, i see alot of people talking about it but never understand what they're talking about


something like this:

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The butane can comes with many attachments, find one that will jam exactly into the syring nozzle (maybe with a bit of encouragement/duct tape/araldite) and you're sorted - push down the syringe until it fills up, then squirt the amount of gas you need into your chamber, voila.
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Unread postAuthor: Eddbot » Wed Jan 30, 2008 12:11 am

when you push it out of the syringe does it instantly vaporize in the chamber?
and where do i get syringes...
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 12:13 am

It will already be a vapour, what goes into the syringe is a gas.

Pharmacies/Chemists normally stock syringes, say you need them for mixing glue or something similarly innocuous.
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Unread postAuthor: Eddbot » Wed Jan 30, 2008 1:48 am

i gotta check the definition of innocuous

oh, i get it, so basically don't tell them i'm gonna make i giant fireball inside a pipe :roll:
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 2:06 am

that's pretty much the idea :D

Judicious use of the phrase "I need it for a school project" can be very helpful ;)
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Unread postAuthor: devilman » Wed Jan 30, 2008 2:38 am

How much butane should be added to the chamber? Is there a calculation of how much butane to add to achmaber that is x" long and y" in diamiter?
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 3:01 am

x -> chamber diameter in centimetres

y -> chamber length in centimetres

[(0.5x) x (0.5x) x 3.142 x y] gives you the chamber volume in mL

[0.04 x chamber volume] gives you the amount of butane you need in mL
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Unread postAuthor: devilman » Wed Jan 30, 2008 6:02 am

if i where to use a metal 3/4 combustion chamber, how could i achieve a spark without frying myself on each shot?
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