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New Semi-auto Gun Idea


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#1 roboman

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Posted 19 August 2008 - 05:38 PM

(Also posted on NerfX)

I just had this idea for a simple semi/full auto gun. The mechanism is very similar to one used by a person on spudfiles, and uses many of the same ideas as Captain Slug's Arr and ABP5K on nerfhaven. Basically, it consists of a short PVC tube w/ an endcap on one side. There is a hose barb mounted in the endcap, which goes to the trigger valve. A piston with a sliding o-ring mechanism is connected to a piece of 1/2 inch brass which slides over a piece of 15/32 inch brass that is connected firmly to the endcap's inside face. The 1/2 inch brass slides inside of the barrel (17/32" or 9/16" brass) and doubles as the bolt. A spring holds the piston in an open position. When air is allowed into the tube, the piston goes forward, stripping a dart out of the magazine, until it slides off of the 15/32" brass and air is allowed into the bolt. The air is channeled behind the dart, shooting it out of the barrel. When the trigger valve is closed, the return spring pushes back the piston, causing the o-ring to uncover the vent hole and allow unused air to escape. The gun will have a selector switch to choose between semi/full auto, and the valve will be activated by a solenoid. The solenoid will be triggered by either a 555 timer circuit configured as a monostable multivibrator or another 555 circuit configured as an astable multivibrator. Both circuits are adjustable by independent trimpots for precise ROFs. The trigger will be a simple snap-action switch that activates the oscillators. The trigger itself is a basic pushbutton valve that opens when pushed, and closes when released. A small LPA tank will be the stock, and will be able to be charged by a small double-acting pump. The solenoid will use two 9v batteries in parallel, to allow longer run times before a battery change is necessary. The rifle will be configured as a bullpup, and will have a 10-round removable box magazine in front of the stock that is expandable to a larger drum magazine.

Does anyone forsee any problems?

(Yes, I know that I forgot to show the oscillators, and they go in between the snap-action switch and the solenoid, with a DPDT slide switch for a selector.)

Posted Image[/img]

Yes, I am new to this forum, and I am only 13, but I can be an asset to this board if you guys give me a chance.

Edited by roboman, 19 August 2008 - 06:21 PM.

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#2 NerfMonkey

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Posted 19 August 2008 - 05:49 PM

This has nothing to do with your design but I like how you waited until your 13th birthday to join. That's awesome.
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#3 Split

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Posted 19 August 2008 - 10:47 PM

That, or the air pressure will just compress the spring and release it all in one shot..
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#4 Renegademilitia15

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Posted 19 August 2008 - 10:51 PM

That is very well written idea thread. When I first saw the title, I thought, "Wow, here comes another half-wit, who thinks he turn a mav into a some sort of nerf cannon." But, I was pleasantly surprised. Very nice idea.

However, since I don't really get into homemades that much, I know nothing about oscillators and 555 timer circuts. I hope this idea becomes reality.

Welcome to the 'Haven.
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#5 roboman

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Posted 20 August 2008 - 01:52 PM

That, or the air pressure will just compress the spring and release it all in one shot..


The oscillating solenoid valve should prevent that.
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#6 CaptainSlug

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Posted 20 August 2008 - 03:45 PM

More complicated than it really needs to be. You can accomplish the same effect using only pneumatic components.
It would be more reliable to have the breech actuation speed controlled by a flow regulator.
And have the trigger valve(s) setup to simply empty the breech actuator into the barrel while closing off the supply line from the tank.

Edited by CaptainSlug, 20 August 2008 - 03:48 PM.

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#7 roboman

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Posted 20 August 2008 - 05:43 PM

More complicated than it really needs to be. You can accomplish the same effect using only pneumatic components.
It would be more reliable to have the breech actuation speed controlled by a flow regulator.
And have the trigger valve(s) setup to simply empty the breech actuator into the barrel while closing off the supply line from the tank.


Unfortunately, I don't have the money/access to the tools for the construction of a full pneumatic system. Also, I am much more comfortable working with electronics.
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#8 CaptainSlug

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Posted 20 August 2008 - 07:56 PM

Unfortunately, I don't have the money/access to the tools for the construction of a full pneumatic system. Also, I am much more comfortable working with electronics.

If you're making the mechanism as shown, the components are pretty much the same. Just have the breech not exhaust at the end of the stroke, but instead have its exhaust controlled by the trigger valve.

You then just have to control the flow rate of the line between the tank and the breech actuator so that it doesn't move too quickly.
The other math you need to do is to determine what load the breech actuator is capable of applying based on the surface area your given pressure level is acting upon (surface area in inches x psi = applied load). Then picking a spring of minimal rating that will retract it after the dart is fired.

The way you have it setup now (if I am interpreting it correctly) you have the trigger valve dumping air into the actuator to move it to a point at which it closes the breech and then reaches a point of exhaust to fire the dart. This introduces a delay between the trigger pull and the firing of the dart that will make aiming difficult. It also significantly restricts the flow rate of the air that will be firing the dart, and that will hurt your optimal performance.

Your system is actually more automatic than semi-automatic, but you don't have enough control over the timing of each part of the feed and firing cycle to be able to adjust the setup to increase reliability.

Edited by CaptainSlug, 20 August 2008 - 08:02 PM.

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#9 roboman

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Posted 21 August 2008 - 01:55 PM

Your system is actually more automatic than semi-automatic, but you don't have enough control over the timing of each part of the feed and firing cycle to be able to adjust the setup to increase reliability.


There are potentiometers on each of the circuits, which control the frequency of oscillation of the valve. A monostable multivibrator only pulses once for a set amount of time each time the input pin is pulled high (or low). An astable multivibrator oscillates continuously while the input pin is high (or low).

See below diagrams:

Posted Image

An astable multivibrator; R2 is swapped with a potentiometer, and the other component values are altered to achieve the desired frequency.

Posted Image

A monostable multivibrator; the resistor is swapped with a potentiometer, etc.
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#10 roboman

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Posted 25 August 2008 - 10:42 AM

Sorry for the DP, but I have made a change to the gun's firing mechanism, after realising that the original mech. was too inefficient (thanks CS). Instead of two separate 555 oscillators controlling the solenoid valve, I will end up using a Parallax Basic Stamp 2 microcontroller. Also, instead of the original breech actuator, I will implement an electromechanical actuator to close a simple breech, which has compressed air run straight through it.

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Edited by roboman, 25 August 2008 - 11:05 AM.

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#11 Split

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Posted 25 August 2008 - 11:00 AM

Do you plan on actually making this? If so, when do you plan on having it done? Pneumatics are not my thing, and do not interest me all that much, so I don't have much to add on that point.
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#12 roboman

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Posted 25 August 2008 - 11:05 AM

Do you plan on actually making this? If so, when do you plan on having it done? Pneumatics are not my thing, and do not interest me all that much, so I don't have much to add on that point.



Yes, I do plan on making this, but it may be a while until its finished, since school is starting up again.
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#13 Guest_DarkInfection_*

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Posted 25 August 2008 - 12:22 PM


Edited by DarkInfection, 23 June 2010 - 09:22 PM.

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#14 roboman

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Posted 25 August 2008 - 01:35 PM

That would work, but I prefer working with microcontrollers, and that's way outside of my abilities, construction wise. My revised design should have a relatively high flow rate, and should be fairly simple to build.
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#15 Guest_DarkInfection_*

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Posted 31 August 2008 - 01:09 AM


Edited by DarkInfection, 23 June 2010 - 09:21 PM.

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#16 InkJet

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Posted 02 September 2008 - 01:09 AM

I'm not sure what kind of pressure you're planning on running this at, but I'd assume you'd need a decently large solenoid and battery to push against the pressure of the spring/air pressure in your little actuator valve, which might be a limiting factor, just sheer size.
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#17 Father Nerf

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Posted 02 September 2008 - 02:50 AM

I know this is a little off topic and I am sorry for that, but is there a reason no one has used the same set up as the Hornet and Big salvo to make a semi auto blaster.
I mean I figure if you added an air tank between the pump and trigger chamber, a small spring inside the trigger chamber, use one barrel and designed a magazine for it, it should work.

Or you could design a Home-made system on the above.

I mean if there is anyone who has ever over pumped their hornet and pulled the trigger they may have noticed the barrel selecter thingy has gone back to the first barrel, and said barrels firing chamber is now full of air again.

Anyway. Sorry for the topic change. Just a thought on semi auto.
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#18 CaptainSlug

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Posted 02 September 2008 - 11:35 AM

I know this is a little off topic and I am sorry for that, but is there a reason no one has used the same set up as the Hornet and Big salvo to make a semi auto blaster.

Those blasters are built around small quick-exhaust valves inside the blast chambers.
Machining the parts to make a Quick-Exhaust valve requires a great deal of precision. I've been tinkering with different part arrangements over the past month and they're very difficult to design and get working properly.

Edited by CaptainSlug, 02 September 2008 - 11:38 AM.

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#19 Father Nerf

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Posted 02 September 2008 - 05:24 PM

Fair enough, I figured the design for those would be difficult. I guess it would be easyer to do with existing parts, so it would be a Mod not a Homemade. I should switch threads. :)
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#20 Guest_DarkInfection_*

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Posted 04 September 2008 - 07:16 AM


Edited by DarkInfection, 23 June 2010 - 09:19 PM.

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