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brass 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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brass gun help

Unread postAuthor: jordzan » Sat Aug 01, 2009 9:21 am

hello folks

the other day i picked up a qev and some fittings and was hoping to get some help on what would be the best chamber size and any other info that might help to make this a better gun, still real new to all this sorry. looking to make a smaller bore sniper rifle, somewhat like an air rifle or small caliber rifle. and ideas criticism advice etc greatly appreciated.

the qev is the style in the pic below in 3/8. was thinking of going down to a 1/4 for the barrel unless that might be a bottle neck ? thinking around 70cm for the barrel 30ish inch's, with a blow gun for the release.

until i can find a suitable steel or alloy bottle, will a well made pvc chamber be ok for now ? i was thinking 40mm 1.5inch pvc 30cm long ?
will pvc thread screw into brass plumbing fittings ok ?

have been trying to make a GGDT file for this gun but it's been epically failing and cannot figure out what i am doing wrong. think it has to do with the chamber inner & outer diam i don't really understand what they mean. is it just to show how thick the chamber material is ? attached a screen shot of the ggdt setup, i know i am doing something stupidly wrong might even be good for a laugh hehe.

Cheers fellas :)

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Unread postAuthor: jmadden91 » Sat Aug 01, 2009 9:23 am

Lol Inner diameter is for coaxial guns. The inner diameter for yours should be 0. At least I think it should haha
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Unread postAuthor: inonickname » Sat Aug 01, 2009 9:26 am

The inner diameter is for modeling co-axial guns. For non-co-axial guns just set the inner diameter to zero.

Also, the stats in GGDT don't seem to reflect what you're talking about.

PVC is fine, if you stick to normal pressures. And yes, if the threads are the same (NPT-NPT or BSP-BSP you're fine).
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Unread postAuthor: jordzan » Sat Aug 01, 2009 10:06 am

Oww i see hehe told you ide do something stupid, i've turned it down to zero and i am still getting the WRN: Barrel potentially choking flow WRN: Barrel choking flow messages ... any idea what i am doing wrong ??

Also, the stats in GGDT don't seem to reflect what you're talking about.

I tried to enter it correctly :( it's all the same as i mentioned in the post
chamber outer diam - 4cm( 1.65inch) Length - 30cm(11inch)
barrel bore - 0.65 (1/4inch) Length - 70cm

PVC is fine, if you stick to normal pressures

what would "normal" pressure be ??

wasn't sure what to do with the piston part so put it as generic with 3/8 seat diameter ... thought leaving it as standard 3inch would cause problem ??
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Unread postAuthor: inonickname » Sat Aug 01, 2009 10:11 am

Ahh.

The seat diameter is wayyy too high for a 3/8" QEV.

Normal pressures would be 100-120 psi in rated PVC pipe.

Just set it as barrel sealing.

Barrel choking flow is insignificant, it just means that the valve has more flow than the barrel.
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Unread postAuthor: jordzan » Sat Aug 01, 2009 10:32 am

when setting it as barrel sealing what would i put the piston diametre as ? figured its about .276inch ish ? even when i change it around the gun still has no power at all. cant figure this out for the life of me :(

i have searched ggdt to try and learn how it works im not being lazy ... just the only idiot who cant figure this program out sorry.

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Unread postAuthor: inonickname » Sat Aug 01, 2009 10:52 am

Try make your projectile a lot lighter, and see what you get.
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Unread postAuthor: Technician1002 » Sat Aug 01, 2009 11:10 am

jordzan wrote:when setting it as barrel sealing what would i put the piston diametre as ? figured its about .276inch ish ? even when i change it around the gun still has no power at all. cant figure this out for the life of me :(

i have searched ggdt to try and learn how it works im not being lazy ... just the only idiot who cant figure this program out sorry.

Image[/img]


Garbage in garbage out. I was looking at your acceleration graph and realized your acceleration is mostly very late in the curve.. Then I looked at the piston mass.... :D

Wow.. Solid lead piston.
No wonder it is slow.. Then I looked at the dead volume. 3.05 cubic inches. May want to revisit your dimensions as start over. Start with the weight of the piston and follow it with the volume of air between the piston and projectile.

To find volume.. Take the cross section of the cylinder and multiply by length. To find the cross section, use Pi times the radius squared.

Your bore is 0.256 inches. Radius is 1/2 the diameter or 0.128. 0.128 times 0.128 is 0.016384. That times Pi.. 3.1416 is 0.051471974 square inches of area. For a volume of 3.05 cubic inches the barrel has about 60 inches between the valve and projectile. :D You might want to try again with the projectile a little closer to the valve. Then you may want to find a closer value for the mass of the piston.

Find some real values instead of garbage values to put in in order to get something besides garbage out.
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Unread postAuthor: jordzan » Sat Aug 01, 2009 11:13 am

the most i can get out of it is 167 ft/s .... with lighter projectile and trying different combinations of barrel length, chamber sizes and psi rating.

is 3/8 qev to 1/4 barrel just never going to shoot well ? not sure if should just scrap this gun and sell the qev etc or ignore the program and just build the gun ?
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Unread postAuthor: inonickname » Sat Aug 01, 2009 11:18 am

Tech, he's obviously using a neutron-star derived projectile also, you'll find that 100g is a bit high of an estimation for a 1/4" projectiles.

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Without even trying, I get that.. ofcourse, you may not be able to build it that well..who knows
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Unread postAuthor: spudtyrrant » Sat Aug 01, 2009 11:32 am

put the piston mass as .5 ounces put the valve seat as .1875 and the projectile as 2 grams and you will get a better result and the dead volume should be under 1" cubed also i got 309fps with your cannons specs

EDIT:oh nvm seems ino got to it before me but the valve seat is .1875 or 3/8"s. woops i'm wrong about that. i just divided 3 by 8 and used that maybe i should check before i post lol
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Unread postAuthor: inonickname » Sat Aug 01, 2009 11:38 am

spudtyrrant wrote:put the piston mass as .5 ounces


Pistons in QEV's are extremely light and small- basically a perfectly shaped piece of rubber. .1 ounce at the most.

Also, the porting isn't exactly 3/8"- that's the imperial thread size. I'd put my money on the porting being basically at least 1/4" or higher, so I wouldn't be concerned too much.
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Unread postAuthor: jordzan » Sat Aug 01, 2009 11:56 am

excellent thank you so much, seing all the mistakes clear as day now.
I've taken the qev apart and measured and weighed all the parts i think i need too.

Just to make sure, have i got the correct idea about the seat and vent diameters in the picture below ?? as too which is which ??
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Unread postAuthor: jordzan » Sat Aug 01, 2009 12:09 pm

Piston mass - 0.1410958oz

according to the pic i just posted, vent = 0.408 seat = 0.591
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Unread postAuthor: POLAND_SPUD » Sat Aug 01, 2009 12:34 pm

in your pic 'seat' is, in fact 'vent' and 'vent' is 'seat'... however, in this particular situation 'vent' is so large that you should put there opening on your pilot valve (a rough guesstimate 3mm for a blow gun)


your seat (marked as vent in your pic :) ) seems quite large... have you measured its ID or OD ?? (you should measure its ID - if you did that that's ok and sorry)
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Last edited by POLAND_SPUD on Sat Aug 01, 2009 3:05 pm, edited 1 time in total.
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