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Still confused on chamber size

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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Still confused on chamber size

Unread postAuthor: Mitchza89 » Mon Oct 01, 2007 7:19 am

G'day lads, I got my new shock pump today. I got my bb gun pumped up to 80 or so psi before my arm just died. I know I can greatly reduce the chamber size but to what I don't know. It's 3/4in copper n I'm using close to a meter of barrel. Anyone got any idea?
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Unread postAuthor: Thursto » Mon Oct 01, 2007 7:38 am

How long is your chamber?
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Unread postAuthor: Mitchza89 » Mon Oct 01, 2007 7:40 am

Ahh geee....Not too sure. It would be close if not over half a meter.
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Unread postAuthor: Thursto » Mon Oct 01, 2007 7:42 am

maybe 30cm woud be good size, possibly i dunno mayb if omene could punchit into GGDT or something? Thats what would do probably.
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Unread postAuthor: Mitchza89 » Mon Oct 01, 2007 7:43 am

Yeah good idea. I'll try the ratio thingo on there.
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Unread postAuthor: Marco321 » Mon Oct 01, 2007 7:48 am

Think about this.
A large chamber has say x amount of air in it. A chamber half the size but twice the pressure has the same amount of air in it. A chamber half the size again but double the pressure has the same amount of air as before.

So by following that logic, you should be able to make a chamber 1/4 of the size, but 4 times the pressure and still have the same amount of air pushing it. But the air will be at greater pressure, meaning more speed.

So if i was you, i would go with 1/8 of a meter, 1/4 of its current size roughly, and pump it to like 300ish.
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 01, 2007 7:52 am

It would be close if not over half a meter.


That's far too large, for a 3/4" chamber 10-15cm is more than enough and you should be able to take it up to 300 psi comfortably. Make sure you alternate between hands though because bulding up too much bicep on one side will have you labelled a wanker ;)
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Unread postAuthor: Mitchza89 » Mon Oct 01, 2007 7:54 am

Thanks alot guys. I really appreciate it. I always thought it was too large. Looks like I was right ;)
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Unread postAuthor: Marco321 » Mon Oct 01, 2007 8:09 am

Mitchza89 wrote:Thanks alot guys. I really appreciate it. I always thought it was too large. Looks like I was right ;)


The first reaction is often the most correct
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Unread postAuthor: Mitchza89 » Mon Oct 01, 2007 8:15 am

Yeah your not wrong. I never go with my gut feelings. I reckon it's about time I should.


BOOYAH! 300 posts :D
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Unread postAuthor: noname » Tue Oct 02, 2007 12:19 am

I used a 1/2" x 8" chamber with a brakeline running through it, and that wasn't too bad. After 5 or 6 shots in a row my arm got kind of tired.
After pumping/sweeping/mopping, all with my right arm, for 2 hours every day for 2 years, my right arm literally looks about 3 times as buff as my left. It's funny! :D

I'm in a strange mood.....
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Unread postAuthor: pyrogeek » Tue Oct 02, 2007 12:35 am

Dude, you need to invest in a CO2 setup.
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Unread postAuthor: Spudnik Tweaker » Tue Oct 02, 2007 1:24 am

Marco321 wrote:Think about this.
A large chamber has say x amount of air in it. A chamber half the size but twice the pressure has the same amount of air in it. A chamber half the size again but double the pressure has the same amount of air as before.

So by following that logic, you should be able to make a chamber 1/4 of the size, but 4 times the pressure and still have the same amount of air pushing it. But the air will be at greater pressure, meaning more speed.

So if i was you, i would go with 1/8 of a meter, 1/4 of its current size roughly, and pump it to like 300ish.
Really? So although there would be the same volume of charged air, the end-resulting velocity of the projectile would be faster? Can someone please confirm this?
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Unread postAuthor: Marco321 » Tue Oct 02, 2007 1:50 am

Spudnik Tweaker wrote:
Marco321 wrote:Think about this.
A large chamber has say x amount of air in it. A chamber half the size but twice the pressure has the same amount of air in it. A chamber half the size again but double the pressure has the same amount of air as before.

So by following that logic, you should be able to make a chamber 1/4 of the size, but 4 times the pressure and still have the same amount of air pushing it. But the air will be at greater pressure, meaning more speed.

So if i was you, i would go with 1/8 of a meter, 1/4 of its current size roughly, and pump it to like 300ish.
Really? So although there would be the same volume of charged air, the end-resulting velocity of the projectile would be faster? Can someone please confirm this?


I'm not sure if this is correct, but it makes sense to me, i reasoned that a chamber size at 100psi, will fire a projectile, as the projectile goes down the barrel, pressure will drop bellow 100psi and could maybe be 20, as the projectile leaves the barrel. I figured a chamber 1/4 of the size, but 4 times the pressure, the pressure would drop from 400, meaning that there will be more force per unit area on the projectile per unit time, so the pressure in the barrel behind the projectile just before it leaves the barrel might be 100psi. I reasoned that this would give a greater velocity.

If someone is willing to do the math and graph pressure as a function of time as the projectile moves down the chamber, that will give a better indication of if I'm right.


Possibly when i finish school in about 1 and a half months, i will do the math behind it all, I'm looking at building a copper rifle, but thats a different story.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 02, 2007 2:01 am

You can easily model it on GGDT. Model an imaginary launcher and keep all the parameters fixed (barrel length, valve spects, projectile etc.) and see what results you get by halving chamber volume and doubling the pressure etc.

I think you'll find that for the same quantity of air, the higher pressure should yield better results.
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