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Thread: 12 ga upper for M-11/9 project

  1. #381
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    Between the length and weight, that's going to create one hell of a moment arm acting against the lower and the take down pin.

  2. #382
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    Quote Originally Posted by bigj480 View Post
    Will it contain any part of a FCG like a hammer? I forget, is this just a one-off project or something you plan to submit to FATD?
    FCG & recoil backstop all contained in the Mac lower. I’m making every effort to make it “approvable” - while I’m building this as a private effort, my ultimate goal is simply to make this capability available to all MAC owners. My contribution to the Mac world. Think of this as a proof of concept.
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  3. #383
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    Quote Originally Posted by theredneckengineer View Post
    Between the length and weight, that's going to create one hell of a moment arm acting against the lower and the take down pin.
    Could you clarify your statement?
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  4. #384
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    Quote Originally Posted by rybread View Post
    Could you clarify your statement?
    From Google:
    A moment arm is simply the length between a joint axis and the line of force acting on that joint. Every joint that is involved in an exercise has a moment arm. The longer the moment arm is the more load will be applied to the joint axis through leverage.


    A good primer video on moments, torque, and force vectors: https://www.khanacademy.org/science/...rial/v/moments


    Think of it as a large lever being used on a fulcrum or pivot point. The longer the lever, the more force on both the pivot point and the "short end" of the lever.
    In this case, it would appear the weight of a full mag and the current barrel/barrel shroud will be rotating about the take down pin on the lower and pushing upwards on the inside rear of the receiver.This will be hard on the registered receiver and the takedown pin holes.
    There will also be a force cross product present pushing outward against the receiver as well.

    This all applies when the gun is fired from the shoulder and no support is given to the gun in front of the pivot pin.
    This changes slightly if the gun is mounted to a pivot point further out, but that won't completely negate the moment already produced by weight and length.

  5. #385
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    That’s more clarification than necessary I’m familiar with the concept- I thought maybe spell check had oddly corrected “momentum” to movement arm, which would have been impressive. In any case, we will see. It will be mounted (primarily) and I have a sound test plan which features a gradual ramp up in load strength & frequent disassembly to gauge wear. I think due to several mitigating factors, the forces exerted on the lower will probably be less than you are anticipating.
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  6. #386
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    Quote Originally Posted by rybread View Post
    That’s more clarification than necessary
    Sorry, you might say it's a.......
    Force of habit.

    badummtissss.......







    Quote Originally Posted by rybread View Post
    I’m familiar with the concept- I thought maybe spell check had oddly corrected “momentum” to movement arm, which would have been impressive. In any case, we will see. It will be mounted (primarily) and I have a sound test plan which features a gradual ramp up in load strength & frequent disassembly to gauge wear. I think due to several mitigating factors, the forces exerted on the lower will probably be less than you are anticipating.
    Neither "momentum" nor "movement" inferred or implied.
    Regardless of the rounds you use, you're still going to face these same issues. The bolt thrust isn't what worries me (for the most part) it's all the other stresses on the receiver.
    In this case the stresses on the top of the receiver, the back plate, and the take down pin holes are all of concern and I can't see anything that's going to "mitigate" them. Add in the twist and deflection of the receiver when being fired and it could add up to a real problem.


    One other issue I see that you'll need to overcome:
    If you're relying on the saiga bolt carrier to push your mac bolt all the way to the rear while in recoil (to the point it can be grabbed by the sear), you're going to have to over-gas the crap out of the gun, and likely run more than one gas port just to get enough gas. The weight of your combined mac bolt and saiga carrier will now be one combined mass that you're going to have to push rearward hard enough to build up enough inertia in the mac bolt for it to be caught by the sear.
    Otherwise, it will short-stroke and you'll have a runaway gun......which is far from ideal.
    Not to mention that over-gassing a gun can cause it to beat itself to death.


    As always,my 2 cents, food for thought, YMMV, no offense intended, carne diem, sic semper tyrannosaurus, see store for details, etc etc.

  7. #387
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    I don't understand how there is a pivot point as all of the force is linear inside of this system? I mean I see how it would strain the retaining pin, but not how there is a fulcrum involved.

  8. #388
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    Quote Originally Posted by Gaujo View Post
    I don't understand how there is a pivot point as all of the force is linear inside of this system? I mean I see how it would strain the retaining pin, but not how there is a fulcrum involved.
    The take down pin on the lower becomes the fulcrum. It's a see-saw with one "leg" much longer than the other.
    The weight of everything forward of the pivot pin produces a force acting on the pivot pin, thanks to good old gravitational pull.
    It's a factor on any Mac upper longer than the original uppers, but it's a function of both mass and length.
    In this case, it's a very long arm and very heavy, comparatively speaking.
    When the gun goes bang, the two sections try to pull themselves apart, which exacerbates the whole thing.
    The recoil is only as linear as the bolts and recoil mechanism/s, which I don't see set up yet so I can't speak to that.

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    Ok... So perhaps a "tab" of steel going under the front of the MAC receiver and takedown pin, back to the trigger guard, to spread the force out a bit? Or going around the bottom and sides and clamping to the rear of the pin? Or clamping/bolting through the trigger guard?

    I'm not an engineer by any means, but in my head it might help mitigate things...

  10. #390
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    Redneck, no offense taken- but you’re pushing it with the puns man!

    This is all valid discussion, as this was a major concern from the start. Here are some of the mitigating factors:

    - Similar to 9mm, a 12 gauge primer requires ~6lbs of force. The recoil spring can be tuned to deliver something close to that minimum- in turn minimizing the required gas force.
    - Overall bolt stroke will probably not be any longer than the Max-15. Additional buffers will also shorten the travel distance.
    - While the upper does not contain any recoil mechanisms there is a catch/ stop for the bolt & carrier, mitigating the recoil force to the lower. The lower is far enough forward that this travel will engage the sear.
    - The side plates may extend beyond the rear of the Mac lower, so the upper hosts the stock. This renders the Mac to effectively be a trigger pack: Most of the recoil force from the upper is conveyed to the mount or the operator vs the take down pin.

    Keep it coming guys. This wouldn’t be possible without your input & support- I appreciate it!
    Last edited by rybread; 02-15-2020 at 09:38 AM.
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  11. #391
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    Got the stock gas valve off successfully- good to know it wasn’t just me- it ultimately required some torch heat but it’s off and the custom valve will go on after some cleaning. I had to mallet out the gas puck also- probably due to the fact I put 5-700 rounds through it before cutting it into pieces!
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  12. #392
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    With the custom gas regulator installed.
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  13. #393
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    Quote Originally Posted by rybread View Post
    - Similar to 9mm, a 12 gauge primer requires ~6lbs of force. The recoil spring can be tuned to deliver something close to that minimum- in turn minimizing the required gas force.
    The recoil spring has almost nothing to do with the required gas needed to cycle the bolt and carrier group rearward. Springs are used for giving the bolt carrier a hard shove back into battery at the end of it's travel, and in most cases, dampening cycle rate. (Obviously, if your spring has an abnormally high K value, such as a truck suspension spring, then that is a different matter because you're not going to compress that spring in the first place.)
    The gas coming through the gas block gives the piston a hard shove that is momentary and serves to give the bolt and carrier the momentum needed to cycle. The heavier the combined objects, the more gas needed to shove them hard enough to fully cycle.
    If you're using the mac bolt to hit the firing pin on the saiga bolt, then you'll be pushing the combined mass of both objects rearward, not just the saiga carrier.

    Go get some stick-on wheel weights and add a pound of them (or whatever your mac bolt weighs) and put them on a normal saiga12 carrier and go shoot it. You will soon find out that the gun that used to cycle fine no longer cycles fully.
    You can even remove the recoil spring entirely if you like and the carrier still will not cycle all the way rearward.

    Short barrels will make the problem even more difficult to solve, but that's an entirely new discussion.

    Quote Originally Posted by rybread View Post
    - Overall bolt stroke will probably not be any longer than the Max-15. Additional buffers will also shorten the travel distance.
    The mac bolt has to have enough travel that it can build up inertia because that inertia is part of what is necessary to ignite primers reliably. Slowly applying 5-6 pounds of force to a primer does not guarantee ignition, it needs to be a sharp, almost instantaneous force if reliable ignition is desired.
    The shorter the stroke of the mac bolt, the less reliable the ignition and the less likely that you will reliably engage the sear with the bolt, ergo a runaway gun.
    As mentioned above, your spring will play a factor as well. Too weak, no bolt inertia and unreliable primer ignition.
    Quote Originally Posted by rybread View Post
    - While the upper does not contain any recoil mechanisms there is a catch/ stop for the bolt & carrier, mitigating the recoil force to the lower. The lower is far enough forward that this travel will engage the sear.
    The recoil force on the lower (from the mac bolt) will be much less than a normal mac bolt because of the issues mentioned above; you'll be hard-pressed to get enough energy into the combined bolt and carrier assemblies to get anywhere close to the same recoil forces as a normal mac.
    Whether your recoil systems are in the lower or in an AR buffer tube like the TASK setup, the lower and take down pin still experiences the same amount of tensile stress, although not directly on the back plate of the lower.



    Quote Originally Posted by rybread View Post
    - The side plates may extend beyond the rear of the Mac lower, so the upper hosts the stock. This renders the Mac to effectively be a trigger pack: Most of the recoil force from the upper is conveyed to the mount or the operator vs the take down pin.
    You're still putting the take-down pin in an excessive amount of compressive stress, albeit less if everything forward of the take down pin is tied to the stock assembly, although that would require tying the muzzle end of the barrel to the stock or something equally silly.

    The problem I see is that all that weight forward of the take down pin is still creating a lot of leverage against everything rearward of the pin. Material deflection alone is going to cause a lot of twist (torsional stresses) and distortion to the upper receiver frames if not properly reinforced. Leverage (due to the weight and length of the large moment arm you've introduced) will stretch and damage the registered lower.



    (If puns aren't your thing, I can always switch to "dad jokes". I have plenty of those!)
    Last edited by theredneckengineer; 02-16-2020 at 02:52 PM.

  14. #394
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    Well I disagree with your initial statement- you have some valid concerns but they are based on some assumptions. The gas is the force that compresses the recoil mechanism so to suggest one has little to do with the other? Engineer?

    You also assume a full sized bolt while also forgetting the hammer spring (again- actuated by the gas force) is no longer present- partially mitigating the additional bolt mass. It should be able to operate at approximately the same energy levels- each shot providing the energy necessary to load and detonate the next round- like the cycling of a semiautomatic Saiga. No more, no less. Just much faster.

    The gas is regulated, so there is no need to open it up until “the gun beats itself to death” only until the required force is achieved- Which is why the custom regulator is so critical.

    Dude no dad jokes, I’ve either heard em all or when I hear it I’ll groan.. maybe just as much as I would for a pun.
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  15. #395
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    Bolt, carrier & magazine all perfect. Cycles snap caps easily. I broke this thing in hard before disassembly.
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  16. #396
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    Quote Originally Posted by rybread View Post
    Well I disagree with your initial statement- you have some valid concerns but they are based on some assumptions. The gas is the force that compresses the recoil mechanism so to suggest one has little to do with the other? Engineer?
    Yes I am an engineer by education and by trade.
    I have no problem being wrong, hell I have been married twice so I'm bloody well used to being wrong...but my stance on this whole thing is not mere theory or assumption.

    I recommend getting a copy of George Chinn's book The Machine Gun.
    Volume 4, page 15 (regarding recoil springs) might be of some use to you. I won't post it here because it's a bit lengthy.

    Yes, the bolt must overcome the minor resistance of the recoil spring but that is negligible compared to the amount of energy necessary to actually push the bolt and carrier rearward and out of battery. Once you have enough force to beat the first part, you should probably have enough momentum to beat the second part.
    Ergo, my comment, "The recoil spring has almost nothing to do with the required gas needed to cycle the bolt and carrier group rearward" is factual.
    A properly designed recoil spring system is used solely for absorbing, storing, and releasing kinetic energy. If you're using a spring as a crutch for poor or inadequate bolt/gas relationship, you will have a heck of a time getting it to run right.


    Quote Originally Posted by rybread View Post
    You also assume a full sized bolt while also forgetting the hammer spring (again- actuated by the gas force) is no longer present- partially mitigating the additional bolt mass. It should be able to operate at approximately the same energy levels- each shot providing the energy necessary to load and detonate the next round- like the cycling of a semiautomatic Saiga. No more, no less. Just much faster.
    Again, I don't assume anything; I'm going by the full-size mac bolt you posted that you put detent pins in, with the idea that it would delay the bolt movement slightly.


    I don't know how you're going to set this up, that much is true, but whether both bolts are under spring pressure or not does not change the amount of energy needed to get the whole thing moving rearward in the first place. Whether both bolts are sprung or not does not negate the fact that you're going to need a LOT of gas to get the whole works moving rearward, and by the time you get enough gas to move that whole thing to the rear......well, don't be surprised if it beats up the gun, or at best, your shoulder.

    Quote Originally Posted by rybread View Post
    Dude no dad jokes, I’ve either heard em all or when I hear it I’ll groan.. maybe just as much as I would for a pun.
    Do Libertarian dad jokes count?
    https://www.youtube.com/watch?v=XMIIahxK5Jk


    Please remember that my commentary is not meant to be insulting or argumentative. I'm trying to make sure your project works right the first time, that's all.
    Last edited by theredneckengineer; 02-16-2020 at 02:53 PM.

  17. #397
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    Totally get it and appreciate it. That’s a stretch regarding the recoil spring- but I get the point you’re making. Again, I’d be careful with assumptions.. that is NOT the bolt being used for this upper- that was an off the shelf stock bolt used only for a proof of concept with the detent system. I actually don’t think it’s going to be necessary at this time but when I’m done with this I’d like to get back to that.

    I had some time to look further and did find volume 4 available online, here:

    https://archive.org/details/TheMachi...e/n35/mode/2up

    Very good reference, thank you!
    Last edited by rybread; 02-16-2020 at 11:21 PM. Reason: Update
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    Glad to see this still going on.

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    Quote Originally Posted by gfunk View Post
    Glad to see this still going on.
    Yes sir. Right now working on fabrication of side plate mounts for the MAC & Saiga components. Now that I have multiple critical pieces in hand, I think it will come together rather nicely!
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