Jump to content

Archived

This topic is now archived and is closed to further replies.

CatgutViolin

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

Recommended Posts

Title says it all, really. I've been working on this idea for the past couple of weeks but didn't want to post anything until I felt confident that I could complete it in one form or another. I took some pictures of my progress so far.

 

The start was basically 'If I make a duplicate of the Martini-Henry barrel from my last project, and align a Saiga magazine in front of it, will an aligned PVC rod be able to feed the shell?'. As you can imagine, the answer was yes.

 

6tQVLDE.jpg

 

I didn't take any pictures of assembling the barrel, but I started work on the receiver by drilling ventilation holes. Ordinarily I wouldn't start with a purely decorative feature, but I figured the chances of screwing up were pretty high so better to get it out of the way first. I then made a cutout for the magazine well and worked out the bolt design.

 

qmlurek.jpg

 

Here's a better picture of the bolt with a shell chambered. Note that the extractor goes on the underside of the gun, so the bolt is actually upside down here. Also, this is the bolt's fully-forward position- the chamber is deliberately undersized relative to the shell, which reduces the overall bolt travel required. The total bolt traverse is 2.5", compared to the shells' overall length of 2.25". I did end up slightly shortening the chamber further, cutting it about in the middle of where the extractor groove is in the below photo.

 

WGSKdj2.jpg

 

At this point I decided to start roughing out the gas system. The only part it's missing here is a hose connector on the back of the valve for the external CO2 rig, but otherwise this is it. Dead simple. The trigger mechanism will be based on a Maruzen M1100 (in reverse), having a trigger bar actuate the valve, with a disconnector to knock the bar off the valve at the end of the bolt's travel.

 

2vCqNEh.jpg

 

Next up was the magazine well. This is mostly ABS, solvent-welded to the PVC, with epoxy putty to fill in the gaps. The magazine catch itself is from a Suomi KP/-31 submachine gun, as is the stock I'll be using, but everything else thus far is homemade.

 

OvpcGEv.jpg

 

EFVOUvr.jpg

Share this post


Link to post
Share on other sites

And here's a quick video with the receiver electrical taped to the stock.

 

 

So, that's my progress thus far. It may not shoot yet, but as far as I'm concerned the hardest part by far (feeding and ejection) is done. In comparison the gas system should be simple.

Share this post


Link to post
Share on other sites

That is insane. Great work!

 

Question: Why does it feed from the top? I always thought that top- or sidefeeding mags were due to unreliable early magazines in machineguns and that they have disappeared once designers learned how to make effective magazines.

Share this post


Link to post
Share on other sites

That is insane. Great work!

 

Question: Why does it feed from the top? I always thought that top- or sidefeeding mags were due to unreliable early magazines in machineguns and that they have disappeared once designers learned how to make effective magazines.

 

I bet it makes extraction/ejection a breeze.

Share this post


Link to post
Share on other sites

That is insane. Great work!

 

Question: Why does it feed from the top? I always thought that top- or sidefeeding mags were due to unreliable early magazines in machineguns and that they have disappeared once designers learned how to make effective magazines.

 

Thanks for the positive comments, and that's a good question. The answer is twofold.

 

1. Because I have a compulsive need to be different and have a real thing for clunky, stupid WW1/WW2 designs. The Madsen LMG, Bren LMG, Owen Gun, Beretta M1918, and Lithgow F1 were all inspirations for this project, but I did also diagram out designs for side-feed and bottom-feed systems, which leads into...

 

2. As Vorpal commented literally as I was typing that ellipse, it makes the extractor/ejection system really simple. The issue with using a fixed firing pin on the APS shells is that reliably triggering the valves requires the firing pin to protrude nearly a quarter inch into the back of the shell. In a conventional extractor/ejector setup where the shell is held tightly against the breech face and pivots on the extractor, the firing pin gets in the way and prevents the shell from pivoting off when the ejector pushes it.

 

Instead, on my design the extractor stands off a quarter inch from the breech face, which does two things. First, it means the rim has to snap past the extractor before the valve can touch the firing pin, which ensures that it doesn't fire until it's fully in battery. Second, it means that on the rearward stroke of the bolt's travel, the shell is basically just getting pulled along for the ride and isn't tightly held in the bolt. As soon as the shell clears the chamber, it falls out, and since the feed direction is opposite the ejection direction the next round coming down the magazine helps push the spent round out.

 

Side-feed would probably have worked with this system too, but all I could imagine was clipping the magazine on a doorframe, so top feed it is.

Share this post


Link to post
Share on other sites

Make it feed from the bottom and forget any kind of stock and you've have something reminiscent of the (alleged) Sterling 7.62 rifle prototype...

Share this post


Link to post
Share on other sites

Make it feed from the bottom and forget any kind of stock and you've have something reminiscent of the (alleged) Sterling 7.62 rifle prototype...

 

Good eye, that's not a coincidence. My initial paper sketches of the action and control layout were based on the Sterling battle rifle prototype, so that's where the ventilation-holed tubular receiver/shroud and protruding barrel come from. Obviously it has deviated from that design somewhat; aside from the magazine feed location, I figured using a rifle stock rather than a pistol grip will make it easier to homemake the trigger mechanism and will be more durable under my usual abuse.

 

I do need to decide whether I want to shroud the ejection port or not, but otherwise the externals are pretty much set.

Share this post


Link to post
Share on other sites

Interesting deign so far. How are you planning to do the firing pin? Or does this use the new gasless aps shells?

 

Edit: I missed that post apparently. With a fixed firing pin you might run into problem with feeding rounds into the chamber. As they slide in, they will hit the firing pin and jam unless the firing pin is retracted in.

Share this post


Link to post
Share on other sites

Yep, that's why the bolt face is recessed, just like on a real open-bolt submachine gun. Since I made more progress and got the bolt just about finished today, I took some pictures to help illustrate the design. First, here's the bolt face.

 

teHENEc.jpg

 

So, the chambering/firing/extraction process goes through four distinct phases.

 

1. The shell is stripped from the magazine and pushed at an angle into the breech. The raised rim of the bolt is what's pushing the shell, keeping the shell away from the bolt face and firing pin:

 

lyBUeDV.jpg

 

2. The shell feeds into the chamber. Now the shell is aligned with the bolt and wholly inside the raised rim, so it drops about a millimeter into the bolt, but look closely- the extractor hook is in the way and prevents it from going any further.

 

6WYfb48.jpg

 

3. The shell hits the barrel and stops dead. The bolt continues traveling forward, forcing the extractor to snap over the rim of the shell. The instant the extractor clears the rim of the shell, the firing pin contacts the valve, and the last few millimeters of the bolt's forward travel ram the firing pin into the shell.

 

VDIuDXa.jpg

 

4. When the rearward stroke begins, the shell's inertia causes it to stay in place as the bolt pulls back, until the extractor hits the rim of the shell and drags it back with the bolt. It's only just behind the extractor, so the firing pin is barely inside the valve channel on the shell, and doesn't stop the shell from pivoting on the extractor and out the bottom of the gun.

 

b71YTjH.jpg

 

As for whether or not it works:

 

 

As you can clearly see in these high-res photos of my crude handiwork, I haven't done any polishing, gap filling, or real fitment, so once I clean it up I expect it won't have any problem ejecting that last shell with gravity alone.

Share this post


Link to post
Share on other sites

well, makes more sense now. youll just have to make sure the shells dont get whacked hard enough that they dont go off before being shambered fully. I have that issue on one of my early designs.

 

if ejection continues to be an issue, you can also put a small stationary ejector in the receiver around where the rear of the magazine fits in. since the cartridges will be tipped up, it wont be in the way of the cartridge being chambered. Doesnt have to be anything complicated either. just a stationary block of plastic that contacts the rim will do the job.

 

I might even start work on my own semi/full auto aps launcher that I drew up plans for around the time I was finishing the bolt action side loading "KS03" thing. My original design had a dual hammer for striking a valve and hitting the firing pin at more or less the same time. But if the recessed bolt works then it would be much easier to just make it an open bolt.

Share this post


Link to post
Share on other sites

I initially included a fixed ejector in the design. The problem is that the bolt needs a cutout for it, but a couple of other elements were substantially simplified by having the pressure-bearing gas tube inside the bolt be the same diameter as the shell rim, so I couldn't cut into it to make a relief channel for a fixed ejector. However, a spring-loaded plunger-style ejector on the bolt face is doable. My plan is to get the gas system operational, and then if ejection is occurring too slowly or not reliably enough I'll add the plunger ejector to assist.

 

I'm curious to see what would prematurely fire the shells. The thought did occur to me that slamming them around at high speed might set them off, but I've been rough working the 870's action at times and never had a slam-fire. I figured a more likely failure point would be the overshot card popping out and dumping the BBs into the magazine or action, but that's solvable with a bit of tape.

Share this post


Link to post
Share on other sites

I'm not really sure of what causes the slam firing myself. The problem occurred mainly in the single shot bolt action "k02" i had made prior to the mag fed one.

 

As best as I can tell it occurs if the shell is fed in at to straight of an angle for too long. On that gun, the shells would slam fire once the front of the shell would engage the chamber. Even though the firing pin was completely retracted into the bolt face, just the force of the bolt face alone was enough to set off the shell. It would even do this with the firing pin completely removed from the bolt.

 

On the mag fed design it also occurred frequently in the early stages of the design that had a similar chamber to the single shot. And the only way I was able to fix it was by lengthening the chamber back towards the magazine so that the shell is still tilted when the front of it starts feeding into the chamber.

 

Thinking about it logically, I guess that sounds like if the shell encounters force on both the front and back, it can jostle the "primer". It doesn't seem to be an issue if the shell is held in the chamber by the rim.

 

One interesting thought about that though, is that it may be possible to incorporate that into an open bolt design, and have the shells fire with no firing pin at all.

Share this post


Link to post
Share on other sites

My immediate guess is that maybe the valve pin/'primer' was sticking out the back of the shell just a tad. It wouldn't take much, only a fraction of a millimeter, for the forward impact of the bolt to strike the primer first and propel it forward enough to trigger. If that's the case, then I think the optimal solution wouldn't be to have no firing pin, but rather a very short one (say, 1mm long) capable of imparting that necessary momentum.

 

I deliberately made the firing pin long enough to fully depress the valve pin to ensure reliable discharge, but if the bolt's momentum is enough to do that with a shorter pin then you could avoid a lot of the design constraints imposed by a long firing pin.

Share this post


Link to post
Share on other sites

I just tested it to see if the valve were sticking out the back a little. Turns out they're actually recessed a bit in when you take them out of the shell charger. So it would seem that is the momentum of the bolt along with the straight feed angle that sets them off. Which makes sense.

 

I'm not sure how well that would translate into a design where the rounds are loaded in at an angle. But, if they have a short distance where they feed straight, then the momentum might be enough to fire them.

Share this post


Link to post
Share on other sites

Can't you just put a slightly larger o-ring on the valve stem so it's more snug?

 

I haven't taken mine apart in a while, but IIRC there isn't an O-ring on the valve stem, it's in the base of the shell. So it might be possible to put a slightly thicker one in provided it had the same OD to fit in its groove. That's a potential option if I run into out-of-battery firing issues.

 

As for loading straight vs at an angle, Brigg, the regular pump-action 870s seem to work fine and I've never had one fire out-of-battery while I was pumping it, and I have not been particularly gentle with mine. Whatever was triggering them in your setup has to be something different from the 870. If it really does come down to a straight feed versus an angled feed, then just using an angled feed is an option, or finding another way to induce friction on the valve stem will do it.

Share this post


Link to post
Share on other sites

I just remembered something that could help. When I use homemade paper wads, if I seat them deeply in the shell so that they reach the valve stem, then the valve has to push the payload forward in order to discharge. On the 870, this caused 'light strikes' where the hammer couldn't hit the firing pin hard enough to push the payload forward and allow the valve to travel. But on this design, the bolt is not stopping until it reaches the end of its stroke or something breaks, so maybe it would be an effective way to solve the problem Brigg describes.

Share this post


Link to post
Share on other sites

Honestly I dont see the premature firing so much as a problem as it is a potential solution to an as of yet un-experienced problem. While slam firing caused some problems in my first bolt action design over a year ago, Ive long since eliminated the problem in the side mag loading version.

 

Im much more interested in the tendency of the shell to fire from momentum as a potential workaround for the complexities of even having a firing pin. That could really simplify a design quite a bit, along the same lines of how straight blowback pistols dont necessarily need extractors. It just removes an unrequired part.

 

Another thought, if momentum isn't enough to fire off a shell, would be that of a weighted firing pin on a weak retraction spring. If the firing pin can gain enough momentum from the forward travel of the bolt to overcome a weak spring, then that force could be applied to the valve in the aps shell in addition to the force exerted by the bolt itself. So it there could be a protruding/retracting firing pin without even having a sear or hammer system.

 

Im not saying that any of it is needed here, but its an interesting thought that I think im going to experiment with.

Share this post


Link to post
Share on other sites

So, you mentioned in my thread, the fact that theres a lot of resistance from the bolt retracting back against the friction of the next loaded shell in the magazine.

 

Im guessing thats maybe the issue youre having with the bolt in your video?

 

One of the ways I was planning on addressing that in my own design was to cut a sort of slope into the magazine-facing side of the bolt. this way theres a little less resistance when the bolt is fully closed and it can pick up a little bit of momentum before it starts encountering friction. If you look on open bolt smgs a lot of them have a similar cut in the bolt, which I can only assume is for the same reason (The Sten especially has a huge cut there). Maybe something like that could help smooth out the bolt's return cycle.

Share this post


Link to post
Share on other sites

Nah, the issue in my video is that at the moment there's nothing holding the gas tube in alignment with the bolt. I was accidentally pulling the valve upwards, causing the interior of the bolt to drag against the O-rings on the gas tube.

 

Interesting thought on the curved bolt, although I'm not sure if that's the intended purpose on the Sten considering some variants of Sten bolt lack that curved area and most submachine gun bolts I've seen just have the bolt cut enough to clear the magazine and extractor.

 

One thing to note that I've discovered is that since the rounds are angled in the magazine, it's possible for the lip of the hull on the next round coming up to catch where the hull meets the base on the shell that just fired, and that stops up the mechanism hard. I was able to fix it for testing purposes by smoothing out both offending surfaces, but I have to wonder if there's a better solution.

 

Currently I'm working on the disconnector/grip design so haven't had any progress to show, but I think I'll go grab some more shells, do a bit of testing re: bolt drag, and report back my findings.

 

Edit: Bolt drag is a non-issue. The real issue is that binding I mentioned. I'm going to try adding a tab to the bolt to deflect the next round slightly and see if that fixes it.

Share this post


Link to post
Share on other sites

Yep, that did it. I had removed the 'rail' on the bottom of the bolt (visible in post #9) and that allowed the hulls to bind as described above. Re-adding it fixed the problem. I'm able to feed and eject up to seven rounds, but the 8th and 9th are experiencing too much resistance to go into battery at 200PSI. If I can't figure out a real solution, I suppose turning up the pressure a bit more will fix it, but I'd like to see if I can sort this out.

Share this post


Link to post
Share on other sites

Triple post: So, the issue with feeding basically boils down to the spring constant on the magazine spring being too high. Cutting down the spring reduced the overall tension, but with the spring constant unaffected the resistance quickly ramps up, and then the bolt has to fight a lot of drag on a full magazine. I could just turn up the pressure and call it a day, but if I can make it work with a lower pressure then that means greater efficiency, reduced wear and tear, and more consistent operating conditions.

 

So, the only realistic solution is to make a new magazine spring. Coil springs are hard, but I can pretty easily cut a sheet of spring steel to a half-inch width, then bend it into a zig-zag to make a flat spring. I did some initial prototyping with scrap I have lying around and got it to work, but I need more steel to make a proper one that isn't held together by superglue, so I have the materials for that arriving Tuesday.

 

Also, I've made some further modifications to the bolt. I finally came up with an extractor design that I feel confident won't break (a bend piece of steel wire, epoxied to a spring steel extractor plate). I also incorporated a makeshift ejector by drilling a hole in the bolt face and inserting a compression spring. With the spring providing a bit of additional ejection force to supplement the next-shell-as-ejector system, the action is able to reliably eject at any angle up to 45 degrees above the horizontal, so it's not reliant on gravity anymore. Ejection also occurs more quickly so it's capable of reliable rapid fire. Also notice that the rib on the top of the bolt has been cut back and angled- this allows the next round to come up just before the bolt clears the magazine, which starts ejection a little sooner at no apparent detriment.

 

opE7yoR.jpg

 

Also, cooldown is proving to be a problem when testing on CO2, so I've ordered an HPA tank and plan to switch over for powering the action.

 

With these changes, it is now reliably feeding and ejecting from a fully-loaded magazine, operates on 200PSI (until cooldown starts to kill it), and doesn't show any signs of excessive wear or damage. I've sketched out a simple trigger/disconnector design with just two moving parts, so next will be building up a fire control group and bending out that new spring when the materials arrive. Also, I'm thinking I may not use the KP/-31 stock after all, since a pistol grip configuration with the grip much farther forward (as on a Sterling) greatly simplifies the trigger mechanism.

Share this post


Link to post
Share on other sites

Made a little bit more progress. Spring steel came in, I made a new magazine spring, and it works perfectly. It has very light tension, just enough to keep the shells in, but offering practically no resistance to the bolt traverse.

 

dTkQMoi.jpg

 

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.

 

mijMfwu.jpg

Share this post


Link to post
Share on other sites

So, quick question. What kind of saiga mag did you use for your build?

 

I just got my saiga mag for mine yesterday. A promag 5 rounder. And it's absolute trash. If you load it and push a round out, the next round isn't pushed up into position, it just gets stuck.

 

And that happens with not just the APS shells, it happens with normal 12 gauge buckshot too. I kind of knew promag were garbage but I didn't think they were this bad.

 

I was trying to get an izhmash 5 round mag but they're sold out everywhere so i settled on this. Unfortunately I can't get mags larger then 5 rounds shipped here =p

Share this post


Link to post
Share on other sites

×

Important Information

By using this site, you agree to our Terms of Use and the use of session cookies.