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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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Semi Auto Nerf Gun

Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 11:48 am

I posted a thread a while back about a game called Humans Vs. Zombies. It's like a real life version of the game Infection if any of you play Halo 3. It's played mainly with Nerf guns. Anyways, After playing the game for several weeks over the summer, I found that Nerf guns just don't pack the firepower needed to mow down a horde of "zombies"

So I recently got to work designing and building my own Nerf gun. I drew some crappy diagrams in Paint to show you before I attempt to explain how it will work.

Gun ready to fire:
Image


Trigger pulled, gun firing:
Image


Next round entering chamber, trigger still held down:
Image


Next round advancing. Trigger released:
Image


Round completely advanced, ready to fire again:
Image



The semi-auto effect will be acheived using a spring-return pneumatic cylinder working in conjunction with a QEV. The cylinder serves as the air chamber for the gun. When air is supplied, it enters the cylinder and causes it to extend. The cylinder is connected to a "bolt" which moves forward to seal off the breech while advancing the next round.

When the QEV is piloted, the air inside the cylinder moves out through the barrel and fires the dart. The spring built into the cylinder causes it to retract. Since it is connected to the bolt, the bolt slides back and exposes the breech allowing a new round to enter the chamber.



The problem with my design is that the bolt starts moving back the instant the dart is fired. If air is still moving through the barrel while the breech is exposed, I may lose some pressure to the magazine. I will try to compensate for that by making the bolt and it's travel longer than the breech opening. Hopefully the breech isn't exposed until the dart has completely left the barrel.



The QEV will be controlled by a 3-way, normally open, toggle control valve. (It functions similar to a slide-check. Air is moving through until the valve is closed. Any air in the output side of the system is exhausted.) Unfortunately it's proving incredibly difficult to get a hold of one of these. I found down a company that can make one for me but it will take a while.

So far I've got all the parts except that valve. I bought an 1/8" QEV off eBay and the seller generously sent me 3! The air cylinder works great and cycles very fast with the QEV. The difficult part when I go to build it will be cutting a neat breech as well as a slot to connect the bolt to the cylinder arm through the 1/2" sch80 barrel. A 1/2" router bit fits well into the PVC so I may use a router. If that doesn't work I'll have to do it by hand with a Dremel.

I will have to make my own darts for this gun because I expect to burn through darts pretty fast and NERF brand darts are expensive. For that I will use 1/2" foam backing rod. The darts cannot have suction cups either, they must have a uniform diameter. I will most likely embed an airsoft bb into the nose of each.

Edit: Here is a video of the cylinder connected directly to a slide check (didn't have the QEV at the time, but it's the same idea)
http://s237.photobucket.com/albums/ff15 ... IM0755.flv
(someone mention to me that this video suggest something inappropriate... haha...)
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Last edited by PVC Arsenal 17 on Sat Oct 18, 2008 12:23 pm, edited 2 times in total.

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Unread postAuthor: Demon » Sat Oct 18, 2008 11:59 am

Well, it look cool but i dont completly understand the cylinder-spring-qev system...

maybe some more explainded diagrams?

do you have to pump at each shot or the cylinder system compresse air to shoot?
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Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 12:03 pm

It will be connected to a regulated air source, most likely CO2.
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Unread postAuthor: Lentamentalisk » Sat Oct 18, 2008 12:06 pm

That is really clever. If I understand you correctly, you are using the Pneu ram as part of the chamber? That is absolutely ingenious. The one thing that could help you not need that expensive 3way valve, would be modding the QEV to fill from the chamber side.
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Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 12:08 pm

Lentamentalisk wrote:That is really clever. If I understand you correctly, you are using the Pneu ram as part of the chamber? That is absolutely ingenious. The one thing that could help you not need that expensive 3way valve, would be modding the QEV to fill from the chamber side.


OH SH!T!!!!!!! I didn't even think of that! Wow that could REALLY make life easier. I will definitely consider it. Thank you!!


Edit: Wait only one problem with that... If there is a constant air supply to the chamber side, the flow of air in may hold the QEV open. What would allow it to close again?
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Unread postAuthor: POLAND_SPUD » Sat Oct 18, 2008 12:36 pm

That is really clever. If I understand you correctly, you are using the Pneu ram as part of the chamber? That is absolutely ingenious.


n fact it isn't.. I posted a similar design (basically the same idea - a 3 way valve + pneumatic ram + QEV) long time ago.... al-xg was the first who thought about using a pneumatic ram as part of the chamber.


I've been trying to build it for quite a long time... the problem is that I can't find pneumatic cylinders rated for higher pressures (~25-30 bar) but I suppose that I'll make a low pressure version first



it's proving incredibly difficult to get a hold of one of these. I found down a company that can make one for me but it will take a while
hmmm are you serious ?? They are one of the most common types of spool valves available

The problem with my design is that the bolt starts moving back the instant the dart is fired. If air is still moving through the barrel while the breech is exposed, I may lose some pressure to the magazine. I will try to compensate for that by making the bolt and it's travel longer than the breech opening. Hopefully the breech isn't exposed until the dart has completely left the barrel.
I doubt there will be any problems with that... inertia of the pneumatic ram + bolt should do the job for you... in case you have anyproblems with it I suppose that you can restrict the flow of air between the cylinder and the chamber with a needle valve or something




I can't wait to see this working - good luck and keep us updated

EDIT
Wait only one problem with that... If there is a constant air supply to the chamber side, the flow of air in may hold the QEV open. What would allow it to close again?
moding a qev is likely to lower the power of the gun... not to mention the fact filling from tha chamber side might prevent the pneumatic ram from retracting after firing the gun
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Last edited by POLAND_SPUD on Sat Oct 18, 2008 12:47 pm, edited 1 time in total.
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Unread postAuthor: Lentamentalisk » Sat Oct 18, 2008 12:38 pm

ya, if you stick a spring behind the piston and put a vent hole, all of the air should go out of the trigger if you hold the trigger too long, rather than the barrel, so it shouldn't fire the next shot. I am actually really inspired by the 'piston as chamber' idea. Thankyou!

edit: well thanks for bringing it back to our attention anyways...

You may want to cut down on all that dead space though... Just raise the piston (or what ever,) so that the QEV can fit right up by the breach
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Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 12:49 pm

POLAND_SPUD wrote:
That is really clever. If I understand you correctly, you are using the Pneu ram as part of the chamber? That is absolutely ingenious.


n fact it isn't.. I posted a similar design (basically the same idea - a 3 way valve + pneumatic ram + QEV) long time ago.... al-xg was the first who thought about using a pneumatic ram as part of the chamber.


I've been trying to build it for quite a long time... the problem is that I can't find pneumatic cylinders rated for higher pressures (~25-30 bar) but I suppose that I'll make a low pressure version first



it's proving incredibly difficult to get a hold of one of these. I found down a company that can make one for me but it will take a while
hmmm are you serious ?? They are one of the most common types of spool valves available



Please understand that I did not steal yours or anyone's idea. This idea came to me a while ago without any outside influence. It really isn't so complicated so it's no surprise that others may have thought of this before.

I have, in fact, had great difficulty finding one of those valves. Maybe I just don't know where to look. Would you mind sending me some links? It would be much appreciated... especially since that's the only thing I have left to buy.
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Unread postAuthor: VH_man » Sat Oct 18, 2008 12:50 pm

I have tried this configuration before.

Unless you have something to fill the chamber back up FAST, its really not very effective for EXTREMELY high ROF.

It does work though, and i was making a marble gun, not a nerf gun, so its definetly worth a shot.
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Unread postAuthor: Lentamentalisk » Sat Oct 18, 2008 12:56 pm

POLAND_SPUD wrote:moding a qev is likely to lower the power of the gun... not to mention the fact filling from tha chamber side might prevent the pneumatic ram from retracting after firing the gun

not so much... The mod really does not decrease the power of the gun by a significant amount.
The real problem is controlling the flow into the chamber. You will need to tinker around with it to get the right ratio of fast refilling, with minimal waste. You will only be able to fill it so fast, given that it has to equalize through the tiny equalization holes (so small to keep the power of the gun from dropping.) Still, I don't think you need it to refill multiple times per second, as nerfs get expensive, or tiring to make if you are going to be shooting rapidfire, just spraying the crowds.
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Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 1:02 pm

I could cut down on the dead space... hmmm...

But to do that I would most likely have to route the air into the barrel through an elbow. I wonder what would deal a greater blow to efficiency- the restriction of an elbow, or dead space.

I should rethink this configuration before JSR chastises me! :P
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Last edited by PVC Arsenal 17 on Sat Oct 18, 2008 1:09 pm, edited 1 time in total.

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Unread postAuthor: Ragnarok » Sat Oct 18, 2008 1:07 pm

While people are arguing - although I'm not going to try to claim I invented it (unlike a lot of you lot), as I was loosely inspired by the operation of pneumatic drills (hence why I dubbed it the Jackhammer) - I actually talked about this method 6 months back, here, and I'd been working on it for a least year and a half before that (Jo makes a mention of my work on it back in April 2007, here), and there are earlier cases of me talking about it on the UKSGC.

The only difference really is that the cylinder in my design is a double acting one with a low pressure fed to the other side, instead of spring return - this was designed as a feature to allow the return force to be fine tuned for best operation.

That said, it's not exactly a complex design, so it wouldn't surprise me if everyone had come up with it independently.

I'm planning on using an adaptation of this idea on HEAL 2, when I get around to it.
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Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 1:15 pm

Ragnarok wrote:That said, it's not exactly a complex design, so it wouldn't surprise me if everyone had come up with it independently.


That's just what I said before. There's not much to it... everyone knows what a pneumatic piston is and I'm sure everyone here could think of some way to apply one to a spudgun. So for every idea, there is an inventor. We all invented it!
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Unread postAuthor: POLAND_SPUD » Sat Oct 18, 2008 1:16 pm

Please understand that I did not steal yours or anyone's idea. This idea came to me a while ago without any outside influence. It really isn't so complicated so it's no surprise that others may have thought of this before.
LOL I didn't said that... I know what you mean the idea is so simple... I am really suprised that I haven't heard of even one working prototype...


AFAIK mcmaster has spool valves... use google.. I am sure that you'll find them
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Unread postAuthor: PVC Arsenal 17 » Sat Oct 18, 2008 1:21 pm

POLAND_SPUD wrote:
Please understand that I did not steal yours or anyone's idea. This idea came to me a while ago without any outside influence. It really isn't so complicated so it's no surprise that others may have thought of this before.
LOL I didn't said that... I know what you mean the idea is so simple... I am really suprised that I haven't heard of even one working prototype...


AFAIK mcmaster has spool valves... use google.. I am sure that you'll find them


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

I suppose the reason not many people have attempted something like this is because it requires parts that fit into other parts with close tolerances- something that is usually only possible if you have a lathe to custom make parts. I was lucky enough to find that 1/2" sch80 is a great fit for Nerf darts AND a great fit for a .40 blowgun barrel which will serve as my bolt.
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