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

Poll for the next XCR Lower project!

3442 Views 66 Replies 8 Participants Last post by  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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What? No AK-XCR lower with ALG (the BEST AK trigger)? 馃槀 JFYI...I'm joking.
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I'm terribly unfamiliar with 5.45x39 but as far as I understand, it uses the same bolt as 7.62x39 but a different magazine? A difference in mag shape would definitely warrant a new lower. It seems relatively niche even for us. I'll see after I refine the process for the AK-XCR, since the design will be similar.



Yeah, I'm attempting to acquire an -M this year but it will have to appear as a target of opportunity; those rifles don't just fall on your lap, frankly. On the other hand, I'm not sure how popular using after market triggers on .308/AR-10 style rifles are. I see some utility but considering the recoil and pressures involved, I figured the default trigger on the -M would be enough. How popular of an idea would this be anyway?

I'm surprised no one is interested in the lower that uses the XCR low-profile trigger, I figured that combo would appeal to the lean rifle crowd. Then again, waiting on RA for parts is a drag and using AR triggers offers the most utility. What brand of AK mag would be most reliable that I can use as the standard template to test off of?
Different bolt too.

AR10s almost all use normal AR triggers.

I'm fine with the XCR trigger...it's not great, not horrible. I think the thought on my end is to try to keep your process as simple as possible. The more changes one makes, the more complexities (and potential for reliability issues) arise.
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I was under the impression that the CZ Bren mags were like AK mags but I see that they have a hole for a mag catch. It looks to me that the lower compatible with Bren mags is gonna be completely different from the lower compatible with AK mags.

Bren magazine, noting mag catch hole and lack of latch on the back:
View attachment 16039
Dimensions from AK-47 stamped receiver, specifically the mag well.
View attachment 16038

Am I getting the right magazine you're talking about?
Yeah, it would be a COMPLETELY different magwell for an AK mag vs. a Bren.

Yugo/Serbian AK Mags have BHO followers and just about any metal AK mag can take the Weapontech BHO followers. On an AK, they just lock the bolt back on the last round....which just tells you the gun is dry. The minute you remove the mag, the bolt slams forward, so you have to insert the mag and rack the bolt to chamber a round; not as fast for most folks as putting in a fresh mag and hitting the bolt release.
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.
Agreed. An AK mag is the draw. It's a completely reliable, inexpensive and ubiquitous solution to the problem of feeding tapered Russian ammo.
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Those are valid points, but truthfully I just want an ak mag lower. That would make me very happy.
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:

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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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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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.
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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Have you decided to go with polymer mags only to avoid having to sleeve the internals of the lower?
No, that was what was available at my LGS and I just needed something that was dimensionally accurate to work with. I ultimately want something that is universally compatible with all AK mags. I never heard about sleeving a magazine well and I've been trying to look it up. I understand that the mags are steel and the upper will be aluminum, so I imagine it will be necessary to prevent gouging. I've never seen it being done though and I can't find any examples yet. Do you know where I can find more info?
That's going to be the problem most likely. The aluminum lower receivers that I can think of that took steel AK mags had to put a steel sleeve in the magwell to get them to work reliably and not wear out the receiver. The Sig 556R was a notable one. M+M Industries M10X uses one too IIRC. I'm sure there are others. That's the problem with an aluminum lower and steel mags.
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Oh, and Magpul Pmags for the AK are probably the most common polymer mags; reliable and inexpensive ($10-12 each). The US Palm's are considered good kit, but expensive often at $30 each.
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You know what? I've decided to keep it simple and stick to making the AK-XCR lower all aluminum and just advise that users use polymer mags with it. It's just that making a steel sleeve that press-fits into the receiver appears to be technically out of my reach in the near and mid-term; with the type of milling machines available to me, I don't see how I can build a steel sleeve accurately and consistently enough to get good results. Steel extrusions would be the best part for this kind of job but I don't have the tech to build my own steel extruder or the money and space to order thousands from a supplier. Instead, if/when I design the swappable magwell lower, I will make the entire AK magwell section out of steel that will pin to the other half of the lower and that assembled lower will gladly accept steel mags.

In other news, I was able to buy the mag release and associated hardware for the CMMG MK47 from CMMG's customer service. Cost me $38 and some change. This'll help a lot with this design job, though how I intend to include one for each of the production lowers isn't something I'm thinking about right now. I suppose I can copy the design of this mag release or something similar when the time comes. Still, it's nice to know that we can rely on a major company to be producing a critical part for this lower.
View attachment 16101
If metal AK mags are a no-go...I wouldn't bother making this lower TBH. You're probably better off designing for the CZ Bren 2 mags.....but that's going to be pretty boutique once again.
.050"?
JFYI, that "beak" is essential to the rock and lock design....it's what the mag indexes and pivots off of.
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On the up side....a steel one can be physically smaller and still stronger than an aluminum one.
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What's your opinion on the trigger hole dilemma? If I use the CMMG part, people can go to them for more mag releases if I disappear off the face of the Earth, but the trigger hole will be smaller. Though, looking at the pictures of the MK47, it doesn't look that bad. If I make a custom part, you'd have to rely on me to get these parts out to people, and I may end up with a small inventory of exclusive parts stashed away in my shop but the trigger hole will be unaffected.

View attachment 16176

I believe the AK-XCR w/ XCR FCG is small enough for a regular AK mag release, but it would only work on that design and AK mag releases can come in all sorts designs and tolerances; I'd never be able to verify that all of them would work. If someone has another idea on the mag release, I'm all ears.
Meh, there's no guarantee CMMG will continue to make the AK mag releases, so I'm not sure anyone can bank on that.
That doesn't look horrible (though you wouldn't be milling the top rail off for a cheek weld. At best, you'd put a rubbery rail cover over those slots).....I bet the 16" will be the hot seller. I wouldn't pay to SBR the shorter ones personally. Well....and now that I look closer....that's not simply an upper swap to a bullpup lower. The ejection port is moved substantially back from the handguard....but I get that it's just a visual representation and photochop of what it would sort of look like.
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