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Marble gun help

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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Marble gun help

Unread postAuthor: Kai » Tue Apr 21, 2009 6:13 pm

Hi, im a noob and i am working on a marble sniper, as you can see in the image below, it isn't completed. anyway, this thing will probably go up to 200 psi. My question is, if i leave it as is by just capping the end of the stock and putting a plug with a shrader in the other tee, would i get good velocity, or would i have to put a bigger tank/chamber on it? By the way, the whole stock is made of 3/4" pvc as of now.


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Unread postAuthor: daniel323 » Tue Apr 21, 2009 6:19 pm

at 200psi you would get some pretty good velocity....

but for safety i wouldn't take your gun to 200 psi.

you could make the chamber a little bit bigger and use 150 psi and have some good velocity.
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Unread postAuthor: Kai » Tue Apr 21, 2009 6:22 pm

so then adding a 3" pvc chamber like i thought i might have to do is un necessary. Also, how much bigger do you think?
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Unread postAuthor: daberno123 » Tue Apr 21, 2009 6:57 pm

If you want to play around with chamber volume and also get velocity figures I'd recommend you download a copy of D_Hall GGDT program. I believe he's even got the specs for a sprinkler valve on his site. You can play around to your heart's content. :D

Here's the link: http://www.thehalls-in-bfe.com/GGDT/
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Unread postAuthor: Kai » Tue Apr 21, 2009 8:48 pm

thats a little too complex for me, could you just give me a recommendation? How well do you think the chamber i have will perform when sealed up?
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Unread postAuthor: jook13 » Tue Apr 21, 2009 9:53 pm

For the diameter of your barrel, your current chamber size is plenty large. Some may argue too big. The common accepted chamber to barrel volume ratio is .8 to 1 For best effiency.

At 200 psi, that thing will probably hold. Is probably good enough to risk holding it close to your body when pressurized? I personally would keep it under 150, but that is me.
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Unread postAuthor: grock » Tue Apr 21, 2009 10:38 pm

jook13 wrote:For the diameter of your barrel, your current chamber size is plenty large. Some may argue too big. The common accepted chamber to barrel volume ratio is .8 to 1 For best effiency.

At 200 psi, that thing will probably hold. Is probably good enough to risk holding it close to your body when pressurized? I personally would keep it under 150, but that is me.


i agree for combustions, but what i always heard was that in pneumatic cannons, more chamber = more power. also, check the PSI rating on the pipe, i think it should hold 200 psi easily. unfortunetly, the sprinkler valve might not, but a piston valve made out of 3/4" or 1" should. and most people block off the stock at higher pressures just for safety, pvc blowing up right next to your face is not fun.
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Unread postAuthor: jook13 » Tue Apr 21, 2009 11:01 pm

grock wrote:
jook13 wrote:For the diameter of your barrel, your current chamber size is plenty large. Some may argue too big. The common accepted chamber to barrel volume ratio is .8 to 1 For best effiency.

At 200 psi, that thing will probably hold. Is probably good enough to risk holding it close to your body when pressurized? I personally would keep it under 150, but that is me.


i agree for combustions, but what i always heard was that in pneumatic cannons, more chamber = more power. also, check the PSI rating on the pipe, i think it should hold 200 psi easily. unfortunetly, the sprinkler valve might not, but a piston valve made out of 3/4" or 1" should. and most people block off the stock at higher pressures just for safety, pvc blowing up right next to your face is not fun.


There are so many factors that change the most effecient CB ratio that it is rediculous. A simple example is with higher pressures, you can get away with smaller chamber volumes. Another example is you can get more power with a shorter barrel using faster opening valves. That is why I used the phrase "common accepted". You will almost always gain a little extra power with a larger chamber. The thing is eventually you will get to a point where the ammo is long gone out of the barrel before the chamber empties. The previously posted ggdt program is a good way to figure out the best figuration.

As to the pressure usage of this cannon, the cannon is only as strong as the weakest link. That being the sprinkler valve you mention. Last I checked 3/4 sch 40 pvc is rated in the 300's in psi. Still, that is rated for water, air is a much more active and violent substance then water, storing much more energy as it compresses like a spring. I stand by my statement of keeping it safe and staying below 150, but it is his call. For all I know, this thing may hold at 800 psi. Maybe.
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Unread postAuthor: Biopyro » Wed Apr 22, 2009 2:32 am

Kai wrote:thats a little too complex for me, could you just give me a recommendation? How well do you think the chamber i have will perform when sealed up?

If GGDT is too complex... wow.

GGDT is extremely easy to use, you just need to ask someone what values to put for your valve and the rest is easy! This will allow you to model and possible combination to find it's power.
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Unread postAuthor: Kai » Wed Apr 22, 2009 3:25 pm

well sorry my 148 IQ isn't good enough mr. i have tesla as my avatar. JK but still, you have to admit, there's a ton of variables
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Unread postAuthor: Brian the brain » Wed Apr 22, 2009 3:36 pm

Kai, we don't mean to flame you.
No need to get sarcastic..okay?

GGDT is a very good tool for just these kind of questions.Reality is, nobody can give you a better answer than GGDT.


But by experience, I guestimate the chamber should be a little bigger.

Too big will not hurt performance, but there is a point it will no longer matter that much.Then it will only take you longer to fill..

I say, build the rest of the stock, it will give you better performance.

Welcome to the forum, and keep it fun!
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: john bunsenburner » Wed Apr 22, 2009 3:42 pm

I should send you a few COMPLEX simulationg programs. Sit down and type in your stuff, ask if you need help(make a chatroom request, or send soem one a pm, or ask in this thread). When you are done save your gun, its a very useful thing to have. Also to be ontopic. I noticed that there is often a certain chamber volume from which on the volume increasing barely matters while a decrease makes a huge difference. GGDT can help you find that "magic number".
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Unread postAuthor: Biopyro » Wed Apr 22, 2009 3:43 pm

There are lot, I agree, and it can be confusing, but when you look at it you actually know most of them. Apart from the valve ones, most are fairly obvious.

I don't know exactly what settings a sprinkler would need, as I've never needed to model one, but I'd say on generic, go for 20% flow coefficient, seat diameter .75, open time 3ms, dead volume 0.
Even if that's wrong, you can compare the change in performance between different chamber sizes.

You have a nice gun there and if you've got this far I have no doubt you can manage GGDT.
As for what size you want, I'd say extend that tee to the same length as the back part of the stock (so it's an F shape) and then put the tire valve in the end of that.
There's no point excessively enlarging the chamber as you'll just have to pump more.
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Unread postAuthor: Kai » Wed Apr 22, 2009 5:39 pm

Okay, I'm trying GGDT again, i need help though

-What is dead volume, flow coef, seat diameter, ?

-I have a 3/4" watermaster sprinkler valve, is that a barrel seal valve?

-And can someone help me w/ projectile data?

-how many grams would an average marble be? i weighed one to be 0.2 oz

yes, i am totally lost

Edit: oh i didn't see that last post biopyro
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Last edited by Kai on Wed Apr 22, 2009 7:42 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Wed Apr 22, 2009 7:37 pm

Kai wrote:Okay, I'm trying GGDT again, i need help though

-What is dead volume, flow coef, seat diameter, ?

-I have a 3/4" watermaster sprinkler valve, is that a barrel seal valve?

-And can someone help me w/ projectile data?

yes, i am totally lost


Dead area is explained in the GDDT documentation. You did read the instructions didn't you?

Dead area is the volume of area between the valve (diaphragm) and projectile (marble). Smaller is better. Your sprinkler valve is in GGDT for it's internal dead volume. Add to it any plumbing volume between the valve and marble.
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