SpudFiles

My FIRST robotics season just ended, and now I'm itching for a new project. I've always wanted to make a shell-loading airsoft shotgun, and I've been successful enough so far with simple friction-held removable-barrel designs to make me want to improve on the concept. I have my own CO2 with Palmer stabilizer and modified QEV to fire semi-automatic bursts of air at constant pressure with a blowgun, all that I need now is a way to swap out shells quickly and reliably enough to make a gun at least somewhat practical for airsoft. So lets get this party started....



My Design Specs

-I plan to use short 2-3" long 1/2" PVC shells loaded with 6mm bbs or 6mm paintballs, wadded with toilet paper
-I may use different shells as long as they're smallish, cheap, easy to make, and not that big of a deal if I lose them
-the gun will use CO2 regulated down to 100 PSI or less
-the gun MUST be safe to shoot at people wearing face protection
-the action must be able to be cycled or reloaded within 4 seconds, the faster the better
-if the mechanism incorporates a magazine, the mag should either hold at least 6 shells, or be able to be reloaded and/or swapped out quickly
-the moving parts must be simple, robust, and, ofcourse, within budget

I'm not willing to spend much over $100, and cheaper is better
I have access to our school's small robotics shop with basic tools and a *manual* mill, so keep that in mind.



So far I've thought of 4 designs: a break barrel, a tube-fed lever-action falling block, a box-mag bolt action, and a box-mag semi-auto.

I'll post pix and details of my current ideas later tonight - I've gotta get out the door.

Seim-Auto / Side Extractor



This is my design permutation of PVC Arsenal's TFS-Semi mechanism. Its mostly the same thing except it incorporates a spring-loaded retention, er, thing, and would most likely use a non-removable magazine.
Shells feed from the single-stack fixed magazine below directly into line with the barrel and valve. Then an ejection rod would simply push out the spent cartridge and retract to allow the next round to come into place. The spring retention thing would have to cover the entire ejection port (not shown), or else the empty space in the bottom right of the breech would mess up the feed (the area directly above and to the left of the sharp inside bend of the mechanism). Unless the shells were cut to fit *perfectly*, some form of sliding barrel seal would be required to slide back and squeeze the shells in ithe chamber, although I'd be okay with a few leaks so long as the system still cycles.
I love this design for its elegant simplicity - there are very few moving parts, and very little things to go wrong. The action would be cycled by pushing the ejection rod into and out of the chamber. This could be done by air cylinder as with the TFS-Semi, or by hand, to create a one of a kind "push-button action shotgun." The fixed magazine prevents the complexity of an interchangeable-mag system, yet would still allow for fast-ish reloads: a groove can be cut in the side of the magazine to allow the follower to be pulled back and held down, then shells could be loaded by lifting up the retention cover and simply dropping them in. Future designs can always be upgraded to interchangeable mags, so I'm not too worried about it.





Falling Block Lever Action

Yes I know falling block is traditionally single shot, so what.

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I thought up this design earlier today, its a bit more complicated but it has potential. Shells are pushed toward the action from a tube mag like in modern pump-action shotoguns. To chamber a round, the lever is brought down halfway to pull down the block, which has a shell-sized hole through it, and the spring pushes a shell into the block when it passes by. The lever is then brought back up, and the gun is fired. To cycle the action, you pull the lever down all the way and tilt the gun back, exposing the fired cartridge which then falls out of the block by gravity. On the way back up, the mag inserts a new shell into the block which is then fired as the cycle repeats.

Although lever action would be undeniably cool, there are a few drawbacks to this design. The block and shells would have to be cut perfectly to fit for the action to properly seal and to allow the loaded block to move up and down without a second shell catching on the lip (like a pin in a lock). The gun would also be limited in capacity by its own length, and a proper reloading method would have to be devised to fill the awkward design.

For the First robotics contest, i want to know, what did you had to build exactly and in language what did you programmed?

Good luck on your project also.
For a shotgun you would only need to use .12 and .2 if you want to reach further ( no need of .4 )

What tools do you have?

u could use a 1/2 qev and a slide valve and get a mag and barrel and bam semi auto shotgun

cant wait to see what you come up with sounds cool

Twizi I agree with you but I think he wants the shells... is that what you were saying? Or were you talking about no cartridges?

no it could be used with shells what i suggest is use 3/4 pvc get some used 12 gauge shot shell and they fit perfect with the 3/4 pvc so make it a break barrel and your done or add a mag system with a spring

Then how would you eject them? Or would the carts just shoot out?

they would either shoot out or with a break barrel he could pull them out
their is a cannon that use this break barrel desighn with a sprinkler valve and a soda bottle i used somthing similar

Ok cartridge ejecting would probably be better since he is using this in airsoft because the shells sometimes stay with the stuff in them and that would hurt

here http://www.spudfiles.com/forums/break-b ... 16580.html

and http://www.spudfiles.com/forums/14-gaug ... 15521.html

and here is a homemade semi auto slide valve http://www.spudfiles.com/forums/semi-auto-t160.html

ejecting cartridges, well here's some examples
http://www.spudfiles.com/forums/deus-ex ... t7723.html
http://www.spudfiles.com/forums/deus-ex ... t8418.html
http://www.spudfiles.com/forums/cartrid ... t7176.html
http://s30.photobucket.com/albums/c328/ ... system.png
http://www.spudfiles.com/forums/tfs-sem ... 19059.html
this video has an intresting pusher system to push cartridges out the ejection port
http://www.youtube.com/user/gronain973# ... NskxbtcYeM

Wow, its good to see such fast replies, sign of a healthy community

Thanks for the links to the cartridge systems, I've already seen most of them but its good to have 'em all in one place.

The semi-auto design I was thinking of is a modified version of PVC Arsenal 17's TFS-Semi, which also happens to be one of the links posted above by iknowmy3tables. I'm working on posting a few pix of what I'm talking about, so just hold tight for a bit.

twizi, I already have a system semi-automatic airbursts, which is why I'm thinking exclusively about ammo loading. Obviously for an autoloader moving parts need to be implemented thru the air system, but thats a given.

Demon, if you're interested in FIRST then head on over to http://www.usfirst.org/, they've got all the info there. My team competes in FRC, FIRST's highschool competition. Every year around late December or early January they reveal the season's game, which is now always 3v3. Each team recieves a comprehensive kit of parts, but you're allowed to get outside parts within a $10,000 budget limit (or something like that). After 6 weeks, you crate it up and ship it to your first contest, so everyone gets the same build time. This year my team (out of pure coincidence team 2468) went to the championships in Atlanta, which is a huge event with 360+ robots. Needless to say, it was awesome even though we didn't place that high.



Just posted my favorite design up top, will post more when I have more time.

Just about everything you said about the TFS is spot on. It is quite a simple design and on paper it would seem that there are very few things that can go wrong... But once you start building it you'll quickly find there are many things that can, and will, go wrong.

Not only do the shells have to be cut squarely for an adequate seal, both the magazine and ejection ports have to be cut squarely. Try as you may to accomplish this, but without machining equipment it's very unlikely.* You'll find that if the lengths of each shell vary even slightly (which they will), some shells will fail to feed/eject and others will just be too loose to seal properly. You will end up having to make the openings larger or cutting your shells shorter just to ensure that all of them fit, and after doing that you'll find that most shells are too loose to seal.

I spent weeks being patient and working to make the gun function as planned, but without the necessary equipment I just couldn't do it. Eventually I broke down and admitted that there was almost no way around having to incorporate a sliding barrel/bolt locking system. Fortunately the TFS design has great potential for that since you're already using a cylinder and a simple linkage is all that's necessary to actuate a sliding barrel.

But... just recently (and with input from members here) I realized that if I'm going to incorporate a sliding barrel/bolt, I might as well scrap the entire ejection mechanism and let the gun itself complete the task by going with a 50% overlapped top-ejection design. This way, the force of the magazine spring pushes the spent shells out and I don't need an extraction rod.

I'm making progress on the first prototype right now and I'll keep you posted on how it goes. But from my past experience with jamming issues, I have little faith in this design's ability to be jam resistant either, despite having seen it in action. A part of me has always wanted to stop dicking around with shells and magazines and instead make a revolver mechanism, but I've yet to find any such mechanism that's easy to build and operate with pneumatic cylinders.

I wish you the best of luck with whatever design you settle on. *Maybe your spring-detent addition to my idea could address the problems I experienced and maybe your craftsmanship (especially since you have access to a mill) is better than mine and you won't even experience those problems. There's also the chance I gave up too soon to stumble upon that breakthrough solution. That said, I would be very happy to see you achieve greater than what I did.

you should just post designs in your replies, that way the thread gets bumped and people realize that there's something new,
I heard of falling block tube magazines for marbles, but for cartridges you'll need a way to eject the cartridges, but if you just want an airsoft shotgun maybe you could use a system that feed loose bbs,

PVC Arsenal 17 wrote: I spent weeks being patient and working to make the gun function as planned, but without the necessary equipment I just couldn't do it. Eventually I broke down and admitted that there was almost no way around having to incorporate a sliding barrel/bolt locking system. Fortunately the TFS design has great potential for that since you're already using a cylinder and a simple linkage is all that's necessary to actuate a sliding barrel.

really what about a blow forward bolt seal? would that reduce power too much?

PVC Arsenal 17 wrote: A part of me has always wanted to stop dicking around with shells and magazines and instead make a revolver mechanism, but I've yet to find any such mechanism that's easy to build and operate with pneumatic cylinders.

that reminds me I hope you also checked out the revo semi so far he's his gun is the only reference I know for homemade auto rotation mechanisms, I'm surprised that you say that pvc arsenal especially with the revo semi