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Semi Auto Nerf Gun

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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Unread postAuthor: clide » Sat Oct 18, 2008 1:25 pm

3-way valves aren't really that hard to get a hold of. McMaster has all kinds of them (pages 958-965). You could mod a blowgun to work as a 3-way; the flow isn't great but it is enough to trigger a QEV. Or use Ebay, a quick search turned up this one, I'm sure you could find cheaper alternatives by looking around. http://cgi.ebay.com/Pneumatic-Manual-Va ... 0292808158

As an estimate to check whether or not your bolt will be opening before your dart leaves the barrel, find the minimum pressure required to move the cylinder all the way forward, then take your chamber volume and barrel volume and use P1V1 = P2V2 to find the pressure in the system when the projectile is at the end of the barrel. If it is higher than the pressure needed to keep the cylinder forward then you don't really have a problem.
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<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult

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Unread postAuthor: VH_man » Sat Oct 18, 2008 1:26 pm

Your using MEGA darts in this thing? (whistles) Dont point that thing at me....

Have you ever made stefans before? If you have any troubles, PM me, I havent bought a NERF dart in 5 years....

For the record, most Stefans are only 2-3 inches long, not the length of normal darts, and secondly, an Airsoft BB in the tip is not going to work at all. You need more weight. What works well is a split shot sinker or .25" steel shot.

Best of luck to ya, I want to see the finished Result. Youve inspired me to make one of these that shoots my specialty Stefans, or NANO's, which fit perfectly in .35" barrel material, and because of their small diameter travel further, faster, and more accurately than the average stefan. Plus, they are smaller :D
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Unread postAuthor: Ragnarok » Sat Oct 18, 2008 1:51 pm

PVC Arsenal 17 wrote:I scoured McMaster for days... the most they have is a 3-way, normally CLOSED valve which is no good for this application.

I suggest something like 4622K53 - It's entirely manual, and it's designed for operating QEVs and pneumatic cylinders.

Basically, there's a sleeve that when pushed forwards connects the supply to the system, and when pulled back vents the system to atmosphere, which I think will do what you need it to. (If not, enlighten me as to what it is you DO need)

I used the same thing (well, one I bought in the UK) to pilot the QEV on HEAL.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: clide » Sat Oct 18, 2008 2:06 pm

Ragnarok wrote:I suggest something like 4622K53...


That is what he is using in the video :wink:

But you do bring up a good point, what is wrong with using the slide valve? You can put a spring on it so you don't have to move it in both directions. The stroke may be a little long compared to a push button valve, but since you already have one it is quick and cheap.
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<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult

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Unread postAuthor: Ragnarok » Sat Oct 18, 2008 2:19 pm

clide wrote:That is what he is using in the video

That would only count if I had any idea the video existed. :roll:
I don't normally back track to the first post in a thread after I've read it unless I see a post that says "Hey, update in the 1st post" - and I didn't, oddly enough.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 3:13 pm

I'm starting to realize that my intended configuration is not so great. I already started playing a few different ones so I may consider using a slide check. It just wasn't practical in my original configuration. The one I have is leaking like hell though...

And yes, I'm using homemade darts. Stefans whatever they call them. If you have any tips I would appreciate it!
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Unread postAuthor: VH_man » Sun Oct 19, 2008 5:45 pm

As for Stefans, I reccomend making them about 2" long. Experiment to find the best length.

For your dart tips, use .25" steel shot for MEGA darts. It works amazingly well.

I assume you know how to make a stefan, but the way that works best for me is to make the "dent" so it goes deeper than your weight will go, then drop it in while the glue foam is still melty, and then cover the tip of the dart SLOWLY, making sure you dont get any glue on the ouside of the foam. If you do it right, the glue should JUST meet the edge, and be perfectly rounded, leaving the foam slightly curiling up around the melting glue....

That, and Make sure your weights are in the direct center, and make sure your foam is cut square.

Also, for Bonus Points, go buy some Colored Hot Glue. Makes them look much more appealing
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 19, 2008 6:10 pm

Interesting take on the "breech held closed while chamber is pressurised" idea, it looks workable.

Have an animation on the house ;)

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Unread postAuthor: kablooie » Sun Oct 19, 2008 6:31 pm

I'm glad to see someone else is using this design, your nerf gun should be awesome.
I've made a bb version, using a homemade ram and piston valve. I had the same trouble as you for the pilot 3-way valve, I ended up using an extremely ghetto 3-way valve made of two blowguns facing in different directions. One is the exhaust and the other was fill, and they are rigged so one is closed while the other is open. It works, just it isn't optimal. And with mine I have no trouble (as far as I can tell) with the bolt retracting from the shell prior to firing, possibly because of the overkill chamber/barrel ratio.

Good luck with this, I look forward to seeing it completed!
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Unread postAuthor: PVC Arsenal 17 » Mon Oct 20, 2008 2:13 pm

Hey thanks for the animation, JSR! What program did you use for that? I could use a simple program for some of my projects.
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Unread postAuthor: SEAKING9006 » Mon Oct 20, 2008 2:36 pm

I use Imageready because I have nothing else. But that statement isn't completely accurate. I use a version of Imageready that went obsolete 7 years ago. :(
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Completed projects:
CA1 SMSS Basic Inline
CA3 PDAB Airburst Cannon

Current Project: Bolt action rifle (25x140mm + 1in shot)

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Unread postAuthor: VH_man » Mon Oct 20, 2008 6:31 pm

This has convinced me to try constructing a PB gun working on this principle.

Relatively Exiting, I may add. My pneumatic Cylinders Greatly Appreciate the use.
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 20, 2008 10:50 pm

PVC Arsenal 17 wrote:What program did you use for that? I could use a simple program for some of my projects.


Jasc animation shop, it's part of the paintshop pro package but I think you can *wink wink nudge nudge* download it separately
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Unread postAuthor: POLAND_SPUD » Tue Oct 21, 2008 6:01 am

so far it looks promising... acctually it's so simple that it's almost 100% sure to work good....

spend more time searching for 3 way valves... I am sure that there are hundreds of places where you can buy them..
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Unread postAuthor: PVC Arsenal 17 » Tue Oct 21, 2008 2:13 pm

I've settled on a simple slide check. I'll stick a spring behing it and make some sort of trigger. Can't get anymore simple than that!
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