Combustion gun blowing cleanout off

[devilman]

Ive built myself a combustion bb gun, its a simple over under designe, it has a 90mm cleanout cap on it. the combustion chamber is 60mm pipe.

I spray some hairspray in there, put the cap on tight, and hit the ignighter. The problem is the cap comes flying out every time. Any one have any suggestions on how i can fix this problem, i carnt fire it till its fixed, cuz it hurts liek hell when it hits you in the arm

Edit- i have made 3 combustion cannons and never had this problem, they all used 100mm pipe

[OuchProgramme]

Is the cap threaded? Or is it just a cap that you squeeze the chamber into.

[devilman]

threaded cap of couse, and i twist it on tight to, and its still blowing it claen off the threads. The other thing is it hasent damaged the thread in the cap or on the chamber,

ill get soem pics if i can find the camra

[elitesniper]

its beacuse your barrel is way to small for that chamber so when you ignite it the pressure goes out that small hole slowly so it just blows the cap off.

[devilman]

Any way i could fix that problem? cuz otherwise i just wasted a fair bit of $ on stuff im not gonna use.

If i put like a hole or something in the combustion chamber to try and release some pressure, woudl that stop the cap comming off? Because here the only sizes they make threaded caps in is 90mm and 100mm. I have a reducer from 90mm to 65mm, or i could have done 100mm to 50mm

[mopherman]

all your parts should be able to take the pressure of combustion in a closed chamber. scrap the back end and get a sch40 plug.

[devilman]

What do you mean by a plug?

SOrry for the dodgy pic but i only have the web camatm, the missus has proper camra. This is what it looks like


EDIT- On another note, i have some small steel pipe (3/4) with a threded cap i could use, i just dont know how to get a spark inside of the metal pipe, ideas?

[sjog]

Seal the cap end and fill it with air.

[devilman]

origionally i had the bb gun run of compressed air, but i got sick of pumping it after each shot, so i wanted to try and make a combustion one, less effort then.

I have changed all my pipes over to 3/4" steel, all i need to do is figure out how to make a spark inside of the steel, any suggestions?

[devilman]

Re-did the gun in all 3/4" pipe, and it was worse then bad. I could propell the lead bb further with my lungs then the combustion could (bb went maybe 1m with the combustion). The 3/4 pipe is just to small to use as a combustion chamber, it has no power what so ever.

So what size pipe should be used for the combustion chamber? should i just go 90mm thread to 90-60mm reducer, then another reducer down to my barrel size. Do this without added extra length with pipe, just enough to go into the fittings

[OuchProgramme]

Change it back to pneumatic

You dont have to pump it after each shot, just have a blowgun w/ a simple gravity fed magazine. Semi-Auto