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Semi/full Auto Homemade

Piston pneumatic (automated bolt action)

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#1 userjjb

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Posted 20 January 2008 - 04:25 AM

Hey guys, been awhile since I've been on NerfHaven. Looks like there's been lots of cool stuff done since last I was here! I got reeled in by the allure of the greater power of potato cannons and Nerf fell by the wayside, but now that I'm in an apartment there isn't much room for 900 fps golfballs anymore. Classes don't start for me for another week and I've been bored looking for a project to work on that will get along with my smaller confines.

Having looked at some of the stuff CaptainSlug has been working on gave me a couple ideas for a very simple semi and/or full auto nerf gun. Essentially it's a fusion of a small piston-valved pneumatic launcher along with a bolt-action breech. The key feature in this design is that the bolt is actuated by the piston, enabling semi and full auto fire.

For an example of the type of bolt-action breech I'll be using check out Retiate's Clipped AT2K.

For several examples of barrel-sealing piston-valved pneumatic launchers, see my archived thread on Spudtech. (The third launcher down uses the same size components to be used in my nerf gun)

For an example of the HPA source/setup I'll be using see CaptainSlug's Paintball Tanks With Nerf, Fun with HPA topic.

Now that that's all out of the way, here's my design:

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There's a few key elements that are omitted/not obvious; In the first diagram the actual magazine reciever and clip are omitted for simplicity. Also it looks like the bolt is coming out where the darts would enter, this is just poor artistry on my part, the bolt would exit through a slot in the 1/2" PVC (see Retiate's write up to see what I mean). Also all of the breech elements are nested. Going outward to inward: 1/2' PVC, 19/32" brass, 9/16" brass, internal breech action. Finally, in the second diagram the far end of the first diagram is pictured (both 3/4" female adapters are the same part). Obviously things aren't perfectly to scale as well.

The cycle proceeds as such:
1. Dart chambered, bolt forward, breech closed, spring extended, piston forward and in contact with internal bolt action, chamber and pilot pressurized, air source open, exhaust closed.

2. Air source closed, exhaust open. The pilot vents to atmosphere and pulls the piston back. This now vents the chamber into the barrel firing the dart.

3. Spring return on bolt retracts and opens breech. Next dart enters breech.

4.Exhaust closed, air source open, pilot fills pushing piston forawrd and sealing. Piston pushes internal bolt action forward closing breech and stretching return spring. Chamber and pilot are now fully pressurized.

And the cycle is back where it started.

The wonderfull thing about this design is I can adjust barrel length and chamber pressure to customize dart range. I picked as an example a chamber 12 ci @ 30 psi, with a 12 in barrel. One of the members on Spudtech developed a tool that simulates the performance of a gas gun given various parameters and outputs expected muzzle velocity, valve cycling etc. The tool is called GGDT and is available for download at the author's website if you guys would like to try it out. Here is a screenshot of GGDT with the info for this gun input:

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You can see from the graph that the dart will exit the barrel 20 ms after firing. I expect the spring to take significantly longer than this to open the breech, so there shouldn't be any gas excaping even though firing and breech opening occur "simultaneously".

The air source on the 3 way solenoid will be normally open and the exhaust normally closed. So by merely pulling the trigger (and thus cycling the valves) the gun will fire and reload, semi-auto. Here's an example of the 3 way solenoid I plan to use. Alternatively the clip could be taken off and dart's loaded manually in semi-auto mode if the user is out of loaded clips.

Alternatively, the ball valve leading to the pop-off valve can be opened. Now once the pilot reaches the pop-off pressure the valve will open and actuate the piston valve. After firing the continued air source feed will reseat the piston valve until pop-off pressure is reached again, full-auto. For an example of a full-auto piston valve launcher see markfh11q's website.

The best thing about this launcher is that it requires at most a dremel (although you could get away with hand tools) and no machining. All parts are readily available at Home Depot (and wherever you get your brass), with the exception of the the HPA setup and the 3 way solenoid valve, which is around $20 (less if you buy one surplus off Ebay). You can even leave out the o-ring and check valve on the piston, it just improves performance, although o-rings are easy to get at Home Depot and you can make your own check valves for about 90 cents (if anyone is interested I can make a write-up on how I make mine). Once built the whole thing can be dismantled for servicing and repair with no tools whatsoever. Also the design is extremely compact with essentially no external components except for the clip and the bolt return spring (which will likely be covered by a half-cylinder of PVC so it isn't exposed to fingers getting caught or the mechanism being jammed).

EDIT: Added two sentences that clarified how the breech elements are nested.

Edited by userjjb, 21 January 2008 - 02:39 PM.

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#2 CaptainSlug

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Posted 20 January 2008 - 11:58 AM

Unless you can set the breech to stay open for a set interval of time, I personally would recommend avoiding a brass slide breech for this kind of system because it has a rather high tendency to chew up darts. This is why I've switched to using a ramrod system in my most recent design.

And just so you know, that "3-way solenoid valve" or directional control valve is going to have a very low flow rate.

Edited by CaptainSlug, 20 January 2008 - 12:02 PM.

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#3 userjjb

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Posted 21 January 2008 - 02:31 PM

Unless you can set the breech to stay open for a set interval of time, I personally would recommend avoiding a brass slide breech for this kind of system because it has a rather high tendency to chew up darts. This is why I've switched to using a ramrod system in my most recent design.

And just so you know, that "3-way solenoid valve" or directional control valve is going to have a very low flow rate.


I could easily use a small QEV (1/8" or 1/4") in place of the solenoid valve if flow was a major concern, I just would prefer to be able to actuate the pilot valve electronically rather than pneumaticly so I have thin wires running to the trigger rather than tubing.

However, please note that the solenoid is not what's providing the power for the launching of the dart, but rather the barrel-sealing piston. The pilot (filling) side of the piston is pressurized and pushes the piston forward against the portion of 1/2" pipe sticking into the 3/4" tee. The neoprene on the front of the piston seals off both the pilot and chamber allowing them to pressurize. Once the exhaust port of the solenoid is opened the pilot area behind the piston vents to atmosphere and the chamber pressure pushing on the outer area of the piston slams it backward. This unseals the barrel and allows the chamber to vent down it propelling the dart.

The flow rate of the solenoid can actually be quite low and still properly actuate the piston valve. Especially with the o-ring and check valve you can pilot the valve with low flow. I have a 2" valve (that can be seen in my Spudtech thread referenced previously) that I can actually pilot with a Schrader (tire/bicycle) valve and that has around a 6 ci pilot volume behind its piston.

I agree however that the most problematic out of everything is getting the darts to feed smoothly, we'll just have to see how it goes once I get everything together. A great many people already uses breeches like this albeit not automated, so I don't anticipate it being to troublesome.

I may eliminate the internal bolt action and just extend the length of the 19/32" brass to reach the piston. I'll have one less part that can break/has to be made. This would also have the side benefit of allowing this piece to be removed from the inner section of 9/16" brass for disassembly/repair.

One idea I've been throwing around is having the opening of the breech actuated by the trigger pull rather than a spring. This again removes additional moving and external parts and increases simplicity of the design. I imagine the trigger-pull as a two stage actuation. The trigger would have some free movement backwards before it came into contact with the breech slide, during this time it would actuate the valve. Once the valve is actuated the breech would no longer be held forward and now that the trigger engages the breech slide it would continue to move backward, opening the breech and chambering a dart. Upon trigger release the pilot of the valve would resume refilling and the breech would close again. This would make the design less jam prone I think, but unfortunately the design would no longer be capable of full-auto. I may try the trigger breech return if the spring breech return doesn't work out.

BTW, CaptainSlug what do you think of the Gas Gun Design Tool (GGDT), have you tried it out yet? I would think it'd be right up your alley. It's a really quick and accurate way of predicting muzzle velocities before you go ahead and make the darn thing. It's accurate to with a couple percent, IIRC the only thing it doesn't take into effect is the transfer of heat from the working gas to the gun, the guy who made it is the Lead Firing Officer at the US's China Lake facility.

EDIT: Threw in some ideas for potential improvements and mades some comments about GGDT.

Edited by userjjb, 21 January 2008 - 03:16 PM.

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#4 CaptainSlug

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Posted 21 January 2008 - 08:31 PM

I'm not particularly keen on math or theoreticals so I don't really have any use for software of this type. And I've already done enough practical experimentation to understand what works, why it does, and what kind of performance I should expect.
Real experimentation is ALWAYS going to teach you much more than you can learn through a simulation. If you're not sure where to start you should find something that works, take it apart, try to figure out why it works, then use it as a basis for an experiment to work towards your end goal. It may take more time, but you'll gain a better understanding of all of the factors involved.

I'm sure others might find it useful, but I don't see myself needing or wanting to use it.

Edited by CaptainSlug, 21 January 2008 - 08:35 PM.

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The little critters of nature, they don't know that they're ugly. That's very funny, a fly marrying a bumble bee. I told you I'd shoot, but you didn't believe me. Why didn't you believe me?


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