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Gas-Operated, Semi-Auto, Magazine-Fed APS Shotgun


CatgutViolin

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I also started on roughing out a pistol grip and receiver assembly. I'm still not yet sure whether I want to use a pistol grip or the wood stock design, but I figured I'd mock up the grip assembly, test the fire control system, and then decide- worst case, I saw off the pistol grip and shove the guts in the wood stock.

 

 

 

could maybe take inspiration from the Owen smg and kinda do both? 

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I managed to get a trigger group together, which means I'm set on the pistol grip, and will be going with a tubular stock (akin to an AC556, but without the ability to fold). I made a quick video showing the operation of the trigger system- I'm particularly happy with how this came out, because not only is it extremely simple, but it could be applied to pretty much any system using gas pressure to propel the bolt.

 

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Just a little bit of progress today- I forgot to upload some WIP images before, and today I roughly assembled the trigger guard. It still needs cleanup, but all the internal parts can disassemble out the front without the trigger guard needing to come off.

 

Here's how I made the grip and internal components. I first drew out the parts in a graph-lined sketchbook, then affixed them to a sheet of 0.25" ABS with packing tape. The scroll saw (a cheap $80 Chinese appliance from Wen) cuts it pretty neatly, and the pattern makes it easy to keep straight lines and follow curves.

 

TwP2LED.jpg

 

The grip itself is three layers of ABS, with the internal components cut out of the middle layer. This made it easy to ensure everything fits nicely and holds together solidly. This picture should make it clear how the firing mechanism works, if the black parts on black frame were hard to make out in the video:

 

cBsy2YF.jpg

 

And lastly, today I made a trigger guard out of layers of PVC pipe:

 

hhvZ068.jpg

 

I haven't done any finishing yet so those ugly saw marks, melted plastic, and other deformation will be eliminated before it's complete. The last functional requirements are affixing the grip to the receiver and fashioning the stock attachment assembly (which will hold the gas system in place). Once that's complete, it's down to just external, non-functional components like the stock, grip panels, sights, gap filling, and cleaning up all the rough surfaces.

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You should try making the grip panels look sorta like MG42 grip panels. maybe out of wood.

 

Its probably just cause the grip frame unit kinda looks like one off an Mg42 already, but I think that style, with all the thin horizontal lines would look good. Especially since youve already got the sort of early 1900's aesthetic going.

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Yeah, MP44/MG42-style grips (raised panels with horizontal grooves) were the plan. I'm thinking I'll do the grip panels in ABS and solvent weld them on, then finish them as either black bakelite or faux wood, not sure yet, depending on how the rest of the gun comes together and whether or not the ergonomics warrant a faux wood foregrip.

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Figured I'd do a quick tutorial on how I did the grip panel.

 

First, while the graph paper template was still taped to it, I used a box cutter and a straightedge to cut along the lines. This put a line every quarter-inch.

 

HxJOvUm.jpg

 

With those preliminary guide lines scored, I went back with the box cutter and deepened the lines.

 

zc0SidQ.jpg

 

Next, I used a small hobby saw to cut along the grooves, deepening and widening them.

 

hC98Cui.jpg

 

Then, I carved out the channels using a triangular steel file. I put the point down, giving it a nice 60 degree angle on either side.

 

fsPp2ge.jpg

 

Lastly, I used some 150-grit sandpaper to file down all the corners and round them off, then sanded the surface to clean it up a bit.

 

7pEhDxf.jpg

 

It's a little messy so I'll need to do some further cleanup on some of the lines, but functionally it accomplishes the intended purpose with very little effort.

 

I also contoured the trigger, bulked up the charging handle, attached the other sideplate, sanded to match the top of the trigger group to the rounded contour of the receiver, and glued it in place.

 

RC28GCJ.jpg

 

The only functional component it's missing is the stock, which is needed to hold the gas unit in place, but that should require comparatively little effort. If functional testing goes well, then it just needs an endcap, sights, a handstop, possibly a foregrip, gap filling, and an awful lot of sanding and smoothing, and then it'll be ready for paint.

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I got the stock unit attached, and then functional testing did not go well. So, here's the current state of the gun:

 

t2pYpx8.jpg

 

Looks pretty much the same as before, except now it has a stock. On the inside, though, I had to make quite a few adjustments. In no particular order:

 

-The barrel geometry was wrong, and not sufficiently stabilizing the shell as the bolt moved forward. To lengthen the chamber, I had to make a whole new barrel. On the bright side, I was able to make a new barrel in about thirty minutes, with no really finicky work, so at least I know that producing more lengths of barrel should be fairly easy. I was planning to make an 18" to complement this 12", but I might go ahead and do a 24" or so too.

 

-I substantially changed how the gas unit seals inside the bolt. Before, as in prior pictures, there was a series of O-rings on the gas tube which formed a not-perfect-but-decent-enough seal inside the bolt body. This proved insufficient, with the bolt tending to 'stall out' halfway through chambering a shell. I stripped the O-rings off the gas tube and inserted an aluminum tube just barely larger than the gas tube inside the bolt. Despite having no O-rings, it seems to work much better, and the gun is proving extremely gas-efficient.

 

-As I expected, the brass pin on the trigger was wearing down the critical surface of the ABS transfer bar, causing the disconnect mechanism to engage inadvertently. Plating the front of the transfer bar with a steel shim solved the problem.

 

-The spring under the transfer bar wasn't reliably returning it to the default position, sometimes causing it to only half-return, and then the disconnect would engage during the trigger pull. I put a stronger spring in, and tweaked the neutral position of the trigger to sit a little farther forward, to put a little less tension on the transfer bar when the trigger is released.

 

-The extractor wasn't able to grip the shells closely enough, and sometimes they were slipping free during extraction. I trimmed down the front of the channel the extractor rides in, allowing it to come a little closer to the firing pin.

 

-The firing pin itself wasn't reliably triggering gassed shells, which turned out to be a protrusion issue. I backed out the firing pin a bit, so it will reach the trigger point of the shells before the disconnect engages, rather than relying on inertia after the disconnect engages to carry the bolt farther forward. This means that the bolt will be impacting the shell at high speed, so I put an O-ring in the chamber of the new barrel, both to help seal the shell against the barrel and to provide a bit of cushion during firing.

 

Anyways, that was all a colossal pain, and amounts to about six hours of troubleshooting since my last update, but now it works.

 

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As of tonight, all functional components are complete. Just cleanup (lots of cleanup), paint, and another barrel or two, and then I'm calling it done.

 

Stock and rear sight:

 

hEGRwXd.jpg

 

Front sight:

 

KZFKluE.jpg

 

The sights are offset to the right, as on the Owen and F1, allowing them to clear the magazine well and still get a decent sight picture:

 

k9RVfI4.jpg

 

All in all:

 

d5Qt8bD.jpg

 

So now begins an awful lot of sanding and gap filling to make it look a little less slapped-together.

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I managed to get a trigger group together, which means I'm set on the pistol grip, and will be going with a tubular stock (akin to an AC556, but without the ability to fold). I made a quick video showing the operation of the trigger system- I'm particularly happy with how this came out, because not only is it extremely simple, but it could be applied to pretty much any system using gas pressure to propel the bolt.

 

*YouTube linky*

I have to say that this system is amazing, and you managed to explain it in a simple, easy to understand way that makes everyone feel included. I am kind of jealous. :unsure: 

This has given me a few ideas of my own I need to realize.

Only thing I'd like to know is if this blow-forward-system could be altered into a blow-back system, so one can make a semi-auto shotgun firing from a closed bolt? I know that would requite some more work with firing pin, hammer, sear and related parts, but that would be incredible. Even though my engineering knowledge is rather limited I would still like to try.

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I have to say that this system is amazing, and you managed to explain it in a simple, easy to understand way that makes everyone feel included. I am kind of jealous. :unsure: 

This has given me a few ideas of my own I need to realize.

Only thing I'd like to know is if this blow-forward-system could be altered into a blow-back system, so one can make a semi-auto shotgun firing from a closed bolt? I know that would requite some more work with firing pin, hammer, sear and related parts, but that would be incredible. Even though my engineering knowledge is rather limited I would still like to try.

 

I'd need to draw it out on paper, but I think the same basic system could work as a blowback action. You're right that it would require some additional parts to drop a hammer (personally I'd do striker-fired, it's significantly easier to engineer), but otherwise it would basically be the same mechanism in reverse. The hard part would be keeping the gas system out of the way of the action, so it would probably work better with a separate gas tube.

 

I appreciate the kudos. I've unfortunately been slightly delayed, as further testing revealed some minor issues (addressed by making a new design of extractor for, like, the fifth time), but it should be actually ready for paint now. I also couldn't help but try out some window cutouts in the mag:

 

Ot0ZDSz.jpg

 

I've got another magazine in a box and three more on the way, so I think five magazines should be plenty once I build new springs for them.

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When I look at this, I wonder how hard it would be to build this in the look of a Sten/Sterling.

 

For the most part, pretty easy. I think I mentioned earlier in the thread that left-feed/right-eject was my original intention, and I could have gone back to that at any point up until the trigger group was installed. The main reason I didn't was that shotgun magazines are absolutely enormous, and that can be a problem both for balance (APS shells are fairly heavy) and for navigating obstacles. The trigger group is where it would have to deviate from the Sten and Sterling, in the former case due to trigger placement and in the latter because the Sterling has no protrusion below the receiver for the trigger group.

 

Well, I should hopefully have this project more or less complete tomorrow. I did some cleanup, primed the PVC, and then applied a few light coats of black matte spray.

 

While I was waiting for the paint to dry, it occurred to me that I'm missing an essential element of a post-apocalyptic-esque firearm, so I had to rectify that immediately:

 

sByeE3R.jpg

 

PFMB357.jpg

 

I took the opportunity to build up an 18" barrel to complement the 12", and test fit the other components to see roughly how it'd look:

 

67IkQ7J.jpg

 

I'm not doing any further work tonight to give the paint time to cure. Tomorrow I'll use my graphite rub technique to give the metal a blued finish, which I expect will work well, since both PVC and ABS are porous. Once that's done I'll paint the grips and bayonet grips with my go-to faux wood technique, with a backup plan of semi-gloss black bakelite finish if it ends up looking unconvincing.

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So, er...any plans to upscale to 40mm?

 

I'm strongly considering it but trying not to get ahead of myself. Annoyingly, after installing the new smart shell valve stems in all my shells, I discovered that the geometries of the valve stems are slightly different and they don't reliably trigger, so I've been doing some further modification to the bolt and disconnector to get it operating correctly. Once I actually have the gun working 100%, do a shooting video, and take it to the local field for live fire testing, then I'll probably start looking into 40mm.

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so I've been doing some further modification to the bolt and disconnector

 

...which has now turned into 'fabricate a new bolt entirely', after the current one suffered critical failure and cracked along its length. Whoops. It shouldn't be too much of a setback, though. The bolt isn't a very complex design, it's mostly just reliant on critical geometries that I can easily copy from the old bolt. This time I'll be using an aluminum tube to replace one of the layers of nested PVC, which should alleviate the durability issues.

 

On the bright side, this is giving the body paint ample time to cure.

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