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Which Lower Reciever would you be interested in seeing built this year?

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Poll for the next XCR Lower project!

3436 Views 66 Replies 8 Participants Last post by  Chowwow, 
Chowwow
New year, new start on a fresh, challenging projects!

Work on the AR-XCR lower is drawing down in terms of CAD work, though the transition to CNC machining will require it's own steps. In the mean time, I believe it is time to move forward with the lessons and tools we've gained over the last year of work and produce a new lower for the XCR family. After some thinking, I've compiled a list of possible projects that I can move forward with, and I would like to know which of these ideas have popular support so that I may focus my efforts on a specific project. Here are detailed descriptions of the projects with pros and cons to each one:

AK-XCR with XCR TriggerAn offshoot of last year's Picatinny-XCR lower using factory-standard AK magazines. This will be a CNC machined lower featuring a rear 1913 rail for stocks, the original XCR trigger group, and a low profile magwell and mag latch for the AK magazine.
  • + Low profile and light design
  • + AK magwell specifications are publicly known
  • - Requires designing proprietary mag latch; one that is longer and in a different position than the standard mag latch used in the AK. Will require trial and error.
  • - No BHO unless Yugo mags somehow work.
  • - Must purchase standard AK mag to ensure compatibility.
  • - I do not personally own 7.62x39 barrel conversion. However, I do own a 6.5 Grendel barrel conversion, which uses the same bolt and enough 6.5 Grendel rounds can be loaded into an AK mag ensure function.
AK-XCR with AR-15 TriggerA variant of the AR-XCR lower that uses AK mags. This will be a CNC machined lower featuring a rear 1913 rail for stocks, the expanded body for an AR-15 trigger group, and the AK magwell and mag latch.
  • + Uses AR-15 triggers! No longer dependent on RA for triggers
  • - No BHO
  • - Requires designing a unique mag latch that is different from the one used in the previous proposed design. It must be longer to accommodate the extra height imposed by the AR trigger group. Position of pivot pin will be different too.
Swappable Magwell XCR lower with AR-15 TriggerA variant of the AR-XCR lower that has all of its features as well as being able to swap out the magwell of the lower from a module that accepts AR mags and contains the BHO, to one that accepts AK mags and contains the mag latch.
  • + Feature rich, one and done solution
  • - Complex, and requires careful machining
  • - Development of this system basically involves developing an AK-XCR w/ AR FCG anyway, with further development on the magwell swap feature added at the end. Might be worth holding back development until other designs are proven.
AR-XCR-MEverything that I've done for the XCR-L, but with the lower for the -M. It will be a CNC machined aluminum lower fitted for the XCR-M that accepts LR-308/SR-25 magazines, a rear 1913 rail and an expanded body for the AR-10/AR-15 trigger group.
  • + Added features for the -M, mainly the option to use your own dang trigger for your -M
  • + Expected design will be similar to the AR-XCR(-L), just with the expanded magwell
  • - Not sure if expanded trigger options are necessary for the -M
  • - Might need to design a new BHO
  • - I do not own an XCR-M (yet), and I need to acquire one and go through the same process as I did on the AR-XCR to ensure the function of the design
Bren-XCR with AR-15 TriggerA variant of the AR-XCR lower that uses 7.62x39 CZ Bren 2 mags. This will be a CNC machined lower featuring a rear 1913 rail for stocks, the expanded body for an AR-15 trigger group, and a magwell accepting the Bren 2 mags.
  • + Design (potentially) will be very similar to AR-XCR and can be made relatively quickly
  • + Similar ergos and functions of AR-XCR
  • - Requires extended BHO (hopefully the same one used in the AR-XCR and AR-XCR-M)
  • - May require longer Mag catch (maybe the one on the XCR-M?)
  • - User becomes reliant on CZ to release these magazines in his country. Will be incompatible with AK and STANAG mags.


This thread will be edited to become the build log for the winner of the poll. As it was in the last thread, I expect to have major CAD work done by the summer, and something built in metal by the winter. Your opinions are welcome!
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roflcopters
Flskunkape said:
Just my 2 cents, the bren mags would be a lateral move and not offer any distinct advantage over the existing stanag x39 magazines, I'm doubling down on my ak mag vote, for the mass availability. Also I like the price point of ak mags much more. $30 mags was a lot of what I didnt like about the existing Robinson x39, I don't think another expensive and hard to find magazine is the right solution (atleast for me), unless one already has a bren2 and is already invested into their magazines.
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The bren mag feeds more reliably than the stanag, and you retain lrbho and magazine release. As far as differences between stanag and bren, it would be the fastest to prototype with some dimensional changes. You can mill an existing XCR lower to fit them. We could probably design a steel lower insert that allows you to swap between bren (no insert) and stanag.

I agree that availability is an issue, but it's the cheapest shortcut in development time and prototyping vs the other options.
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F
Those are valid points, but truthfully I just want an ak mag lower. That would make me very happy.
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Sean K.
Flskunkape said:
Those are valid points, but truthfully I just want an ak mag lower. That would make me very happy.
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They are valid....but the Bren mags aren't widely available and are expensive. I'm not even sure what the reliability/durability is considering the Bren 2s in X39 had lots of problems out of the gate (though I don't know that the issues were at all mag related either). Have those issues in the X39 Bren 2 even been corrected? Last I read, CZ wasn't importing (or was it building) them b/c of the problems.

Sounds like they still have problems for a fair amount of owners:

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Sean K.
Why does this even exist?

I can't go to HB's website to see....blocked here at work (I don't have a cell phone, lol).

Were the X39 CZ mags unavailable at some point?

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Sean K.
Then there's this:

Trigger Gun barrel Air gun Gun accessory Wood


Shit, for $295....it might be worth buying a Bren 2....takes AK mags and uses AR triggers. If the gassing problems with the Bren 2 in X39 were solved...this would be a viable option.

I think one thing is fairly certain....the Bren 2 mag option would look better than AK mags in an XCR.
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roflcopters
Bren issues were in the gas block - they are warranty replacing the gas selector, or you can buy aftermarket or drill to the new size.
Replacement parts for the bren are unobtanium, though. I haven't bought mags in a while so I can't speak to that.
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Chowwow
Sean K. said:
Then there's this:

View attachment 16047

Shit, for $295....it might be worth buying a Bren 2....takes AK mags and uses AR triggers. If the gassing problems with the Bren 2 in X39 were solved...this would be a viable option.

I think one thing is fairly certain....the Bren 2 mag option would look better than AK mags in an XCR.
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Lingle Industries be nipping at my ass with these. The only reason why they don't make their lowers for the XCR is because they (still) don't have an FFL. Only a matter of time though. Speaking of which, I chose to keep the rounded edges of the lower for the most part due to the simplicity of using a radius endmill to mill out the edges. If people prefer the geometric edges that Lingle Industries use in their lowers, I can try my hand at that style - though I don't think it fits the style of the XCR that much.
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Sean K.
Chowwow said:
Lingle Industries be nipping at my ass with these. The only reason why they don't make their lowers for the XCR is because they (still) don't have an FFL. Only a matter of time though. Speaking of which, I chose to keep the rounded edges of the lower for the most part due to the simplicity of using a radius endmill to mill out the edges. If people prefer the geometric edges that Lingle Industries use in their lowers, I can try my hand at that style - though I don't think it fits the style of the XCR that much.
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Agreed on the aesthetics...that trigger guard to mag release is pretty hideous.

I do wonder if the XCR is even on their radar considering this platform has what, 14K Ls in existence?
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F
Sean, you sent me down the bren rabbit hole again. I then got less excited when I couldn't find any available other then a couple of 9" ones, paired with having to monkey with the gas plug, it seems like it has a lot of feature that the Robinson has but I feel like cz "missed it by that much" with the bren.

Although lingles lower is finally in-stock, been looking at them off and on for a year plus and I felt like every time I would look they were always out of stock. I had also been eyeballing their stribog lower but had issues with putting 2k-ish into a 9mm build. When I could get a real rifle for that kinda $$$.
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F
No more new voters? I know the community is small but are all the new members bots?
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Chowwow
Flskunkape said:
No more new voters? I know the community is small but are all the new members bots?
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I started a poll on the r/RobinsonXCR subreddit and got 9 more votes (though there is some cross-talk between here and there). As expected, there were some differences in opinion (note I had to vote for one in order to see the results, turns out it was otherwise the most unpopular choice so far).

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My thoughts on the results so far: I'm trying to maximize my efforts by serving the option that provides the most utility to people. The Bren-XCR is intriguing and might be easier to implement, but the relative difficulty in finding the mags benefits a niche within a niche. If I had my own machine shop and FFL, I'd be happy to sell 5-10 a year to order (and probably deliver faster than 6 months vs RA), but we lack that convenience right now. I'd like to do something for the -M rifle, once I get an -M rifle to monkey around with and that might take until next year. A swappable mag feature sounds cool in theory, but as I've seen in the recent week, it's more important to get the functional ideas down first before over-reaching with gimmicks. @Sean K.'s comments on another thread on this idea has also watered down my enthusiasm for this idea, though not totally. The real big issue that I face with building anything with an AR trigger is that I've recently discovered a limitation with mating the two together in my main build log: the AR trigger group is either too far away from the firing pin by nearly .2" or the BCG is too short. I'm working on a couple of solutions now but it's clear the scope the the project has changed. With the AK-XCR projects, at least they avoid that issue by allowing me to move the trigger group up to that distance deficit without having to worry about where I'm going to put the BHO or mag release arms - because they aren't there.
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roflcopters
Simplest fix might be to turn a new firing pin and make the rear thicker so the hammer can reach it. You might run until an issue with the firing pin peening the back of the upper and negating the bcg buffer on higher gas settings
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N
WRT Lingle.

I asked them about a xcr lower awhile back they stated I was not the first to do so. I got one of their stribog lowers(the one before the current generation). The bho(the extended hbi) was a little finicky and seemed to come down to the pivot pin and how far it was in. It didn't have abnormal play but if I held it in all the way the bho worked great. I ended up getting an A3 that's been great.
I do have the latest version of the ghm9 lower and it's nice but even after asking about psa mags working in it, they're all real tight. I've sanded down the front tab on a couple and they work fine but not real happy I have to fit all my mags. The magpul mags are good.
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N
Chowwow,

Is the .2" for the height? That's been talked about before and Alex mentioned it way back about the lower would have to have been taller with AR triggers
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Chowwow
navalbeaver said:
Chowwow,

Is the .2" for the height? That's been talked about before and Alex mentioned it way back about the lower would have to have been taller with AR triggers
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.2" is about the distance from the face of the hammer to the firing pin that I estimate based on visual comparison. The height of the lower has been compensated already. The actual distance might be less in real life, but the issue is that the hammer is not actually hitting the firing pin correctly. I'm working on a solution now; I'll make a lower that a cross-section of the model and see where the hammer lands when the trigger is pulled.
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N
Got you
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Chowwow
First look at the CAD for the AK-XCR. Despite being based off a mostly finished model, it looks very rough. The dimensions of the AK mag are really long and I still need more space for the mag latch. The point where the front of the AK mag begins is the same as the previous design that used STANAG mags, so the magazine should be at the right spot for the bolt to strip off cartridges and load them into the chamber. The ultimate decider on what design I come up with will hinge upon whether people can buy mag latch for the CMMG MK47 or any other extended length mag latch in bulk.

Air gun Trigger Gun barrel Gun accessory Metal
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N
FWIW, I'll buy at least one just for the effort.
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Chowwow
navalbeaver said:
FWIW, I'll buy at least one just for the effort.
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Thanks mom, I knew I could count on you!

Reddit poll results:

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This project will go slowly while I do research with CMMG about that mag release and work with getting an AK mag to work with. Whether I can find a mag release for the smaller AK-XCR will determine if/when I can get to production on it.
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Chowwow
My initial ideas for the mag latch for these AK mag projects is to use a standard AK mag release for the smaller lower that uses XCR FCG and the CMMG M47 mag release for the lower for the AR FCG. abran007's first lower seemed to use a custom release that looked longer than a typical AK release, but I'm starting to see that the shorter AK release might work on the low profile lower. The issue is that the design challenges for each ideas are different from each other and one is gonna have to take priority over the other as to which gets developed first. I'll look around and see which parts are available to me to decide which design I want to dive right in first.

An interesting idea someone on fosscad on reddit designed was to use a standard AR mag release in a rotated position to act as a mag release for an AK magazine lower. I can try doing this for the AK-XCR, though I would like to know what the community's preferences with this were.

Rectangle Auto part Fashion accessory Engineering Font


Gun accessory Rectangle Gun barrel Cylinder Engineering


In other news, I got a US AK polymer magazine from my LGS. I'm hoping this magazine is good enough to use as a model for the AK magazine lower project. Any opinions on my magazine of choice?

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