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GGDT request.

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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GGDT request.

Unread postAuthor: ALIHISGREAT » Tue Jul 15, 2008 6:46 am

now the weather is relitivly good, and i'm on holiday, i have decided to finish my cannons :shock: and i have changed a few of my ideas/plans to accomodate some new ammo i came across (7.5g steel bbs)

so could someone please GGDT these stats

1/2" QEV with blowgun pilot
35cm of 15mm copper chamber (~13mm id) at 200psi
100cm barrel 15mm with 13mm id
shooting a 13mm 7.5g steel bb

Thanks
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Unread postAuthor: Hotwired » Tue Jul 15, 2008 6:57 am

15mm is 13.5 - 13.6mm bore

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Unread postAuthor: Ragnarok » Tue Jul 15, 2008 7:00 am

As a ball park figure, 110-120 m/s, guessing some numbers.
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Unread postAuthor: ALIHISGREAT » Tue Jul 15, 2008 7:29 am

thanks, so that would give me ~30 ft/lbs and room to increase that if i up the pressure.
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Unread postAuthor: POLAND_SPUD » Tue Jul 15, 2008 8:22 am

if you have not bought a QEV yet consider buying a larger size of it (3/4") that would give you more power

it won't be much more expensive.... the difference betwen 1/2" and 3/4" is not that big when compared to increase in performance that you get - expecially if you have to pay shipping
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Last edited by POLAND_SPUD on Tue Jul 15, 2008 8:30 am, edited 1 time in total.
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Unread postAuthor: MaxuS the 2nd » Tue Jul 15, 2008 8:24 am

Use a 22mm chamber too. It just looks cooler.
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Unread postAuthor: ALIHISGREAT » Tue Jul 15, 2008 8:30 am

i already have the QEV as i was planning a smaller bore before i discovered the bbs.... and buying 22mm copper would mean spending my money :shock: and i don't like spending money..... it hurts me....

although now i have upped the bore size, a larger chamber could be useful..

i'm thinking, a 50-100cm 22mm chamber with a 100-150cm 15mm barrel, a 1/2" QEV etc. etc.
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Unread postAuthor: koolaidman » Tue Jul 15, 2008 8:36 am

hotwired, that looks accurate except for the flow coefficient. I dont think 1/4" qevs yeild 45.
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Unread postAuthor: POLAND_SPUD » Tue Jul 15, 2008 8:36 am

how much do they cost ?

well there are smaller sizes of BBs like 10mm (3.5g each) - but for a given configuration you'd get more power using 12.7mm.... but on the other hand 12.7mm BBs are more expensive than 10mm ones

BTW try to make GGDT work on your computer. It's not practical to ask people to check something...
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Unread postAuthor: Ragnarok » Tue Jul 15, 2008 9:00 am

koolaidman wrote:hotwired, that looks accurate except for the flow coefficient. I dont think 1/4" qevs yeild 45.

45% is about the right sort of region for a QEV or barrel sealing piston.

If I'm modelling a piston valve, I'll tend to go for 40-45%.
HEAL's specialised piston valve needs to be modelled as 50% though, otherwise the predicted velocities tend to be a little low - that little valve gives an astounding flow - it's got roughly the same Cv as two 1" sprinkler valves combined.
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Unread postAuthor: ALIHISGREAT » Tue Jul 15, 2008 9:41 am

POLAND_SPUD wrote:how much do they cost ?


£5.95 for 8 bbs from this source http://www.thegeomagstore.com/shop2/product_info.php?cPath=51&products_id=173&osCsid=cb1dfae59f8f511e2b5ac72cab0fc816

and you get 12 metallic blue rods as a bonus...

(i just found some of this stuff lying around in our spare bedroom..... haven't got a clue where it come from.... but i can't complain... its free ammo!)
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Unread postAuthor: Ragnarok » Tue Jul 15, 2008 10:01 am


I suggest you look more in-depth. If you look on the right sites, you should be able to get 1/2" steel balls (which is essentially what the geomag kits include) for much better value.
For that, you're paying nearly 75p a ball.

I suggest a better store might be here: http://www.bearingboys.co.uk/Steel_Ball ... QgodZTJxUw

You can order the 100 pack, and get them for 31p a ball if you go with one of the cheap postage options.

If you want more, you could order 500 and get them for 23p a ball (partly 'cos postage is free if you're ordering that much).

Even if you're ordering small numbers it's probably still more cost effective if you get above about 20 balls.
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Unread postAuthor: POLAND_SPUD » Tue Jul 15, 2008 10:41 am

I've just checked that site... there has to be some place that's cheaper... :shock: becasue I can get 1000 X 10mm BB for 150 PLN, which is about 36.5 GBP

so even if you take into consideration that stuff in Poland is generally cheaper - the shop is still quite expensive...

looking for a shop that does not charge so much seems like a good idea since you don't like spending money :wink:


while browsing the site I found this
http://www.bearingboys.co.uk/Sil-Air30- ... r-12773-p# WTF! is this a fridge compressor ?
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Unread postAuthor: Hotwired » Tue Jul 15, 2008 11:24 am

Haha, looks like it :P

Spruce up an old compressor, weld it to an air tank and... that'll be £425.25 please.
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