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Coaxial Piston gun endcaps?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Coaxial Piston gun endcaps?

Unread postAuthor: Benster » Fri Feb 06, 2009 12:40 pm

Ok I searched for this and got like a 3000 hits, none of which seems to answer my question(s)

I am building a coaxial piston gun, and Have a normal end cap, I have seen that several of the guys on here who have made these, seem to use screws to seal the end cap in place, so that it can be removed for piston maintenance. I was wondering if using one of those thick rubber connectors with hose clamps on both the cap, and the chamber would be safe by itself, or if I should add screws also, if I use screws, do I use ones long enough to enter the chamber? will they leak?

Also, how much room do I need to leave for the piston to move back and forth? I'm sure it could work if I left only enough room for the piston to move back a 1/4 inch, but is more space to be desired?

Thanks for all of the help in advance!
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Unread postAuthor: Cosmic Muffin » Fri Feb 06, 2009 9:11 pm

if your using a rubber connector, it would have to be pressure rated, but i wouldn't recommend it, because it would probably swell under the pressure.
if you use a normal end cap the use screws that go through both fittings and have about 1mm to spare on the other side, this is so it wont interfere with the piston movement.
i have about 1cm for my piston to move back, but im not sure about the other guys.
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Unread postAuthor: tghhs » Fri Feb 06, 2009 10:58 pm

Well i can't help with the screws but i try and have as little room for the piston to move back, say 1/2 cm as to reduce pilot volume
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Unread postAuthor: sniper hero » Sat Feb 07, 2009 2:24 am

isn't half a cm a little too small
all the air has to go past the piston and the sealing edge of the barrel there has to be enough space for a decent flow right?

and I guess that making it on the "regular" way is one of the best :wink:
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Unread postAuthor: tghhs » Sat Feb 07, 2009 2:27 am

sniper hero wrote:isn't half a cm a little too small
all the air has to go past the piston and the sealing edge of the barrel there has to be enough space for a decent flow right?

and I guess that making it on the "regular" way is one of the best :wink:


op's forgot to say my barrel is 1/2 inch
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Unread postAuthor: Benster » Sat Feb 07, 2009 8:26 am

Now talking about having minimal space behind the piston, what about having space that the piston can't travel? I'd like to run some pressure hose from the T at the back of the tank, to my valve, about halfway down the gun, will the cubic volume of air be a problem if I do that?

Also, can I use a schrader valve only, on the filler side, or will I end up setting off the gun when I pull of the filler nozzle and the little puff of air that normally escapes does?
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Unread postAuthor: inonickname » Sat Feb 07, 2009 8:31 am

Benster wrote:Now talking about having minimal space behind the piston, what about having space that the piston can't travel? I'd like to run some pressure hose from the T at the back of the tank, to my valve, about halfway down the gun, will the cubic volume of air be a problem if I do that?

Also, can I use a schrader valve only, on the filler side, or will I end up setting off the gun when I pull of the filler nozzle and the little puff of air that normally escapes does?


Depends on how large it is, and your pilot volume. In a fair sized gun, probably not. You could always add a ballvalve as a safety measure. Consider that a schrader valve is a poor pilot at such scales.
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Unread postAuthor: Benster » Sat Feb 07, 2009 9:12 am

Here is a rough diagram of my plan.
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Unread postAuthor: inonickname » Sat Feb 07, 2009 9:25 am

That will work fine. The schrader shouldn't trigger it. The shorter the length of hose is before the pilot valve, the better. Also, blowguns work better than ballvalves in this context.
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