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Help with semi auto nerf cannon

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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Help with semi auto nerf cannon

Unread postAuthor: brad0896 » Tue Dec 10, 2013 9:03 pm

Hey guys. I would like some help from on a little project I am doing. I play airsoft and at the events we play at, you can use an airsoft "LAW" that fires the 2" nerf rockets to take out vehicles. I have a general idea of what I am doing but would like some advice from you guys. I plan to use a QEV valve and slide valve for the firing mech. I plan to hook up a guerilla air 22ci 3000psi hpa tank with a palmers low pressure regulator hooked up to the QEV for a power source. I will have a firing chamber size of 14 cubic inches (8 inches of 1.5" pvc) and a barrel length of 16 inches (2" pvc) I want to be able to shoot the rockets around 100 feet. I am trying to keep over all size and weight down while maintaining the "LAW" look. I will be stuffing all of this into a 3 or 4 inch pvc pipe for looks. Would the set up I have work? I am worried that the QEV valve will not let out the air fast enough. Thanks in advance! :)
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Re: Help with semi auto nerf cannon

Unread postAuthor: MrCrowley » Wed Dec 11, 2013 2:07 am

A 3/4" QEV will be perfectly adequate for a design like this, you should have no trouble shooting 100 feet. To correct your diagram, the slide valve needs to go between the regulator and the QEV, not sure if that's what you planned but just thought i'd mention it. You'll also want to make use of a PVC street elbow to minimise the distance between the PVC chamber and HPA tank so everything can fit inside a smaller diameter pipe (I think you'll need 4" diameter pipe for this).

If you feel up to it, try play around a bit with GGDT (Gas Gun Design Tool) to optimise your launcher:
http://thehalls-in-bfe.com/GGDT/

You may find you can make the cannon more compact or more powerful for a given size. For example, a 1.5 foot long 1.25" chamber may be just as good as a 14" long 1.5" chamber but allow you to fit the whole cannon in a smaller diameter shroud pipe. I strongly suggest you read up on some GGDT threads on Spudfiles as well as on the site itself and learn how to use the program. It is very simple when you get the hang of it and so incredibly useful for when designing a new cannon.

If you have trouble using GGDT, you can always ask one of us to help model your design for you:
spud-cannon-related/topic23256.html
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Re: Help with semi auto nerf cannon

Unread postAuthor: brad0896 » Wed Dec 11, 2013 6:27 pm

Thank you so much for your help.The link to the GGDT helped a lot! I actually came up with a bit more complicated design but it will be able to utilize a 2.5" outer "Shell" of pvc pipe. I also am downgrading to the guerrilla air 13CI hpa tank as it is only 2" in diameter. I have one more question though, as this will dictate my final design. Does the slide valve have to be attached directly to the QEV? Or can there be some additional piping in between the two so I will be able to bring the slide valve outside of the outer pvc "shell?"
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Re: Help with semi auto nerf cannon

Unread postAuthor: MrCrowley » Wed Dec 11, 2013 9:36 pm

You don't have to have the slide valve directly attached to the QEV but you do have to watch how much space you put between the two. The more space (pipe), the larger the volume of air behind the valve. The little slide valve has to exhaust this volume of air pretty quickly for the valve to work properly, so increasing this volume of air by too much can drastically lower the performance of your gun.

I suggest attaching the slide valve directly when testing the performance on the cannon, if it is adequate then you can try experimenting with different lengths of pipe so that the slide valve is outside the PVC shell.
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Re: Help with semi auto nerf cannon

Unread postAuthor: brad0896 » Thu Dec 12, 2013 4:40 pm

Ok thats good news. I found a 3/4" QEV on ebay. http://www.ebay.com/itm/Pneumatic-3-4-S ... 35c3c58507 I also found a 1/2" slide valve on ebay http://www.ebay.com/itm/360661862600?_t ... EBIDX%3AIT
Will these two items work together? I know I need a 3/4" to 1/2" adapter but I am unsure of what kind. I was also looking into getting the plamers female regulator stabalizer to fit onto the hpa tank. Their website says that the regulator has Female .825x14 NGO threads. I thought paintball tank threads were 1/2"x14 BSP. Will this regulator work for my application?

Edit: I found this adaptor. http://www.ebay.com/itm/BRASS-ADAPTOR-1 ... 1e83c31209
Would this work for the qev and slide valve?
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