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Brigg

Gas blowback, piston driven APS shotgun KSA-04

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So, with Catgutviolin working on his semi auto APS shotgun, I felt like I should get off my butt and start working on my own. I had a couple of designs I had drawn up around the same time as I was finishing the side mag fed ks03, but I had wanted to design a small GBB gun ( which turned into the ksmg1) and really understand the workings of a gbb first before getting into another shotgun. Some of the designs I came up with originally were ridiculously complicated, others too simple to actually work.  

 

The one that I decided to go with is one of the ones I had discounted as being overly complex. however, with some improvements to the design, it has become much more simple. The design as it stands now, will be a open bolt (Most likely), piston driven design with the driving piston sitting over and linked to the bolt.

 

Catgutviolin's design, he mentioned, is a blow forward type with a gas system derived somewhat from the Maruzen M1100 (I assume by blow forward he means that the gas pushes the bolt forward to chamber and fire the shell with a return spring to eject and return the bolt to the rear). My design on the other hand is loosely derived from the operation of a Tippmann paintball marker. Though not laid out in the same way.

 

The bolt is held back by a return spring (I will be trying various types), when released, it goes forward, strips and chambers a round and fires it. As this is happening, a piston linked to the bolt is travelling down a tube on top of and parallel to the receiver (which will be welded to it eventually) . In its first inch or so of travel the piston enters an even smaller tube linked to the gas reservoir and travels there as the bolt moves forward, In the last inch or so of the bolt's travel, the piston engages a valve stem which opens the gas reservoir. At the point which the shell fires, the valve is open far enough that the pressure forces the piston back. Gas is still expanding in the smaller tube, building pressure, and the piston pulls the bolt back along with it, ejecting the shell and returning the bolt to the cocked position. As the piston exits the small gas tube, the excess gas vents out.

 

The piston has a sliding link to the bolt to help ensure that the bolt always closes first before the blowback occurs.

 

Now, as for magazines, I havent decided yet if I will be going to use the magazines I made for the KS03, or if I will just use Saiga or similar available magazines. I might be forced to use saiga mags as my totally straight ks03 mags can only hold 4 rounds before the rims start causing feed problems. And 4 shots is fine for a bolt action, but just isnt enough for a select fire weapon.

 

But, since the magazine issue is in the air at the moment, I decided to start with the gas system. One big thing was that I wanted to completely avoid a remote line. I wanted everything contained on the gun. As it turns out it makes sense on this design, as the gas will be held in a reservoir over the barrel. Like where the cocking tube on a G3 would be.

 

I have already gotten the basis of the gas system and bolt assembly working now, though dimensions may change. I uploaded a short video of it in operation here:

 

 

And Ill post some more pics tommorrow when its not so late.

 

The gas system doesnt look like it has much force, but it is reactive to how much force hits the valve. As can be seen, just the bolt dropping under its own weight with no assistance is enough to propel it all the way out of the receiver. I tested tapping it with a mallet and the bolt was shot hard into the ceiling. So once theres a return spring itll have some nice oomph.

 

Testing the system like in the video resulted in just around 300 cycles. But Im hoping i can get about 20 good cycles once its finished, since there will be a mainspring and ejection to deal with. More would be nice of course but ill be happy if its around there.

 

The receiver is also temporary, just some scrap parts i duct taped together (The main tube is a reject from the ksmg1 build). But once i have the exact dimensions for the gun worked out, ill go ahead and make a proper one.

 

So, theres the beginnings of my take on the semi auto APS idea. More pics as it develops.

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Very nice start! I actually started my design with a virtually identical system, using the 12g CO2 adapter that comes with the APS guns as the gas reservoir/power source, but I found that it took too much force to open the valve. Are your gas reservoir and valve a custom job or adapted from something commercial?

 

Edit: By the way, I found that the Saiga mags are seriously oversprung, and feed reliably with the spring cut to about 3/5 of its original length. That leaves enough room to cram a 9th round in, so the capacity is pretty significant. Like you said, for select-fire you'll probably want a large magazine.

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So far, everything is custom made. If I could have found off the shelf fittings I would have used them but I figured it would be faster to just machine the parts then it would be to wait for shipping.

 

Right now the system uses green gas and the cylinder has a volume of approximately 9 cubic inches. But I had also thought that green gas might not have enough pressure from the start, so I sized the cylinder to accept a c02 cartridge just in case.

 

From looking at a couple of co2 powered valves I noted that the valves are usually very easy to open, so I made mine require very minimal force.

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I had the (foolish) idea of using the valve pin on the CO2 tank/adapter as the actual 'firing' valve, and those are definitely not easy to open. I ended up abandoning that concept mostly because I couldn't find any off-the-shelf parts that would be able to do what I wanted, but since you can machine pressure-bearing vessels you have a lot more options. I imagine green gas like you're using will suffice- realistically, you probably don't need all that many shots between refills. Looking forward to seeing more.

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Awesome. The gas system sounds pretty straightforward and robust.

Reminds me that I seriously need to get working on mine...

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I had the (foolish) idea of using the valve pin on the CO2 tank/adapter as the actual 'firing' valve, and those are definitely not easy to open.

Those are purposely made stiff so you won't end up spilling gas if the tip gets depressed in your range bag. See if you can swap out the spring with a lighter one.

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So I'm *pretty* sure that blowback isn't going to be an issue. I was thinking I could use the return spring from a a WE m4 as the return spring, so I modified the mock up receiver to be long enough for one.

 

The blowback set up was easily able to overcome the spring. But, now my concern is loading. I'm not 100% sure that the spring will be able to strip and chamber a cartridge.

 

What I might do is add some steel weights to the bolt to give it some more mass. But I'm not sure yet.

 

I decided to just go with saiga magazines because they're easy to get. The other option was mags from that MASS underslung shotgun but I heard those mags are very flimsy. I should have a magazine arrive by Tuesday so I can start finally getting into the real guts of the design.

 

I'm debating making the lower receiver from delrin just since it will be so much easier then making an aluminum one but I haven't really decided that yet. Delrin might be easier but all of my other builds so far have been aluminum. I'm not sure I want to change that, but since I plan on it being all black anyway there doesn't seem like there's any reason not to.

 

The fire control will be fairly basic, just an open bolt type trigger and sear set up. I do want to have it be select fire so I have to put together a simple design for that. Which shouldnt be very hard. It will at least keep me occupied until the magazine gets here.

 

I have a weird idea in my head to fix a long mag well and charging handle assembly to the left of the receiver tube, so I can have a non reciprocating charging handle.

 

So overall, still a lot of stuff to do, but I think this will be one of my faster builds. Maybe not as fast as the ksmg-1 though. That thing practically flew together.

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But, now my concern is loading. I'm not 100% sure that the spring will be able to strip and chamber a cartridge.

 

Yes, from my testing this is a significant issue and worth experimenting with ASAP before you commit to a design. If you do go with Saiga mags, cutting down the magazine spring like I said will help a lot, but it still takes a significant amount of force to strip the shell from a full magazine. Also keep in mind that on the return stroke, your blowback system will be working against the friction between the bolt and the next round in the magazine.

 

For your design, I would suggest giving the bolt at least an inch of travel before it reaches the magazine, so that it can build up a little momentum before it hits that shell. I would also suggest you set up the blowback to deliberately overgas it, then have a vent hole in the gas tube at the intended rearmost point of bolt travel; that way it'll run reliably against abnormally high resistance without having massive overtravel under normal conditions.

 

I'm excited to see how this build progresses, it'll be interesting to see the respective merits of our differing designs.

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Oh, yeah Im not *that* worried about extraction/ejection. The piston creates a very significant recoil force and should easily be able to pull the shell past the magazine. And I wasthinking to lock the bolt as far back as possible, hopefully at least an inch but maybe more. Though there is a sort of window where the kinectic energy will be the highest  so ill need to find that sweet spot.

 

as far as the loading, Im thinking I might wind up replacing the magazine springs with ones from an AR15 or G3 mag, since I have a metric buttload of those and I can experiment with them. I imagine a G3 mag spring should be roughly the right dimensions. I can even make some customized delrin or uhmwpe followers if I need to. But thats going to be the trick. 

 

Ill need it to feed as smoothly as I can since interrupting the bolt too much will slow it down. Thankfully the blowback valve is so sensitive that I shouldnt have to be too precise. though I might need to polish the magazine feed lips like I did on the ks03.

 

I think I decided to just do an aluminum lower though, mostly because the delrin I thought I had in 3/4" turns out to be 5/8" and im not ordering more just for this. But I do have plenty of 3/4" aluminum plate laying around, both 6061 and fortal. Besides, I got the delrin in order to make a bullpup rifle design so Id rather keep it for that, since it will probably be the next project after this.

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I'm gonna put this on hold for a little bit, pending the availability of decent magazines. Right now I'm having a lot of issues with the promag type, it just won't push rounds into a ready position, so I'll have to wait to get a decent one. In the meantime I'm going to finish the small bits and bobs on the ksmg1.

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