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Discussion Starter · #1 ·
I've been tooling up and researching ways to design and produce an aftermarket lower receiver for the XCR-L. The thread for AR aftermarket triggers on the XCR and the complete failure of that project has inspired me to go down this route. I see a lot of need for modularity and aftermarket support for this platform, an I'm looking to make this lower be the key stone to all of those bits and accessories that this community has struggled to get over the years.

The plan is to redesign the current XCR lower to support AR aftermarket parts in the FCG, stocks and trigger guard, in that order of importance. The primary goal is to rearrange the FCG to accept AR trigger parts, followed by building in a rear 1913 rail in place of the XCR stock receptacle for potentially infinite stock options as well as allowing for ease of conversion into a pistol without any need for caps, and finally providing accurate holes to allow for trigger guard installs, where possible. The final build will be CNC machined out of 6061 Aluminum and cerekoted.

A 3D printed prototype will be used for fitment testing but a 3D printed lower will not be produced. Such a thing would be a different design with different goals, and I've determined that the XCR is a poor platform for designing a 3D printed lower, due to there being only 2 contact points securing the lower to the upper (only one take down pin and a weak connection using the recoil spring as a tab) versus a typical AR lower. There is not much security there to prevent the bolt carrier and recoil spring from breaking out of a plastic printed lower.

Below is a comparison between the current lower and a mock up of the proposed lower. The dimensions shown are the entirety of the research I have done on the lower so far, more needs to be done.

Trigger Gun barrel Air gun Gun accessory Font


I am well-versed in CAD using Autodesk Fusion 360 and I am now using it professionally. F360 is versatile enough to design this lower, and my thread on Machining Projects should demonstrate that I'm able to program and operate a CNC machine. I'm waiting for a desktop CNC machine to arrive that should be big enough to make this lower. I also now work at a production company that has a CNC machine that I have access to, so come hell or high water I do plan to get this done.

My main issue that I really need help with is getting dimensional drawings for the original XCR lower. It's proprietary information and I doubt Robinson Arms is willing to send me a copy of their blue prints. Right now it's just my XCR-L lower, my digital calipers, dimensional drawings for the AR lower on the internet, and my own two eyes (the same ones that I shoot with) that are being used to design this thing. If anyone can help get me accurate dimensions for the lower outside of the FCG, it would make this project a hell of a lot easier. Otherwise it's going to be a lot of measuring and trial and error.

One issue I found out in the making of the mockup is that the XCR BHO/Bolt catch will need to be replaced with a longer one that spans the extra height of the new lower if we want to have a trigger guard. The BHO system is the heart and soul of the XCR lower, and replacing it with an AR BHO seems to be a betrayal. Still, this means that using this lower requires a proprietary part that Robinson Arms doesn't produce.

I will be quite busy with work and two other personal projects that I've put ahead of this one, but this one is the most complex project with the most amount of time invested so far and the one I need the most help with at the moment. I don't want to set expecations too high, and we'll hold off on future plans beyond this. I expect this to take a whole year and that's not including the last quarter of this year where I'm getting other things out the door. Right now, I'm building it for myself and my own use, but once I iron out the kinks I'd like to distribute this info to the community and get manufactured goods to people who want this lower one way or another. I'm really committed to this project and confident that it'll work out, but the bulk of the development comes with getting those dimensions. I don't need any funding, I just need measurements. Feel free to post ideas and thoughts too.
 

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Needs to be 7075, not 6061.

No, RA won't send you engineering drawings.

If you decide to make a steel AK mag compatible lower, I might be in.

Good luck!
 
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Discussion Starter · #3 ·
Needs to be 7075, not 6061.
I see. I will need to adjust my speeds and feeds but it should be fine. What is your take on a CNC machined lower versus a forged lower? Forging would save a lot on scrap metal and be somewhat easier to machine but it's hazardous to run a forge at my house at the moment.

If you decide to make a steel AK mag compatible lower, I might be in.
What is it about the AK mag that doesn't fit well with the mag well of the XCR? I know there have been issues with certain AK mags on the XCR but I never understood what was the deal with the fitment issues.
 

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CNC should be fine....

The XCR has never taken AK mags. Alex has been against it for 7.62x39 since the beginning. We've had at least one (maybe 2) members that machined XCR-L lowers to take AK rock in mags. Worked fine from his account, but was so much work he didn't want to do them on the side and wanted to sell Alex the rights/licensing to build it from what I remember.

The original Masada/ACR offering was going to have an AK lower available...but it never got off the ground once it got sold to Freedom Group.

Alex prefers the STANAG mag b/c it's faster to reload....problem is: the reliability from ASC and CProducts x39 mags has been VERY spotty over the years. I got lucky and have 22 or so 30 rounders that work, and bought like 11 or so of the 20 rounders (that I've yet to test). They are an odd shape to conform to the x39 tapered case and are cumbersome in a chest rig. I'm currently running the Unimags for x39 and while they seem to be pretty reliable, I don't have 100% confidence in them (and I do have to run one higher gas setting than with the x39 STANAG 30 round mags...which results in more recoil and slightly slower splits due to that).

I'd wager some of Alex's hesitance is that a dedicated lower would have to be made for 7.62x39, then 5.45x39 as well....if there was demand. That, and he's just psychologically opposed to rock in mags.
 

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My confidence in the unimags is growing. The key is to test them all. The bad mags are consistently bad. The mags that work, just work all the time. I'll probably dox the owner and ship all the bad mags directly to his house. The mags that fail seem to have some flex to the mag body like the tack welds/stamping isn't done right.
 

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My confidence in the unimags is growing. The key is to test them all. The bad mags are consistently bad. The mags that work, just work all the time. I'll probably dox the owner and ship all the bad mags directly to his house. The mags that fail seem to have some flex to the mag body like the tack welds/stamping isn't done right.
Agreed. The problem I've had with the company that makes Unimag is when I had consistently bad ones, they wouldn't answer emails or phone calls to get them warrantied. I ended up order 7 new ones (think I bought like 21 or 22 originally) to replace the ones that had a problem. Then when the bill came, I disputed the charges. The CC company never got a response from Unimag about the dispute.
 

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I remember that. It's probably what I'll end up doing but I'm going to send the guy the mags with a note to fix his shit anyway
 

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Discussion Starter · #8 ·
I'd wager some of Alex's hesitance is that a dedicated lower would have to be made for 7.62x39, then 5.45x39 as well....if there was demand. That, and he's just psychologically opposed to rock in mags.
Do you know where I can get dimensions or prints on the AK mag well?

I have to be careful about feature creep on this project and I just want to set the goals as simple as possible at the moment. It's enough work just getting the dimensions on the regular XCR-L lower right now and we haven't even fine tuned it yet. Speaking of which, how tolerant in terms of position are AR FCGs and still work? I mean I've seen the dimensional drawings, but how much room for error do you think I have in the placement of the FCG for the XCR?

Also, does anyone have any particular fondness for aftermarket trigger guards on ARs? The trigger guard on the XCR is comfortable for me and looks wide enough, but I haven't tried shooting it with winter gloves or anything yet. Are aftermarket triggers important to anyone or worthwhile to implement on the XCR?
 

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I remember that. It's probably what I'll end up doing but I'm going to send the guy the mags with a note to fix his shit anyway
Good luck. I tried that several different times. Kinda felt like I was doing something wrong when I reordered with the intent to keep them and dispute the charge, but he really left me no choice.

Honestly, he seems to have a product that works....most of the time. It's unfortunate they have such non-existent CS.
 

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Do you know where I can get dimensions or prints on the AK mag well?

I have to be careful about feature creep on this project and I just want to set the goals as simple as possible at the moment. It's enough work just getting the dimensions on the regular XCR-L lower right now and we haven't even fine tuned it yet. Speaking of which, how tolerant in terms of position are AR FCGs and still work? I mean I've seen the dimensional drawings, but how much room for error do you think I have in the placement of the FCG for the XCR?

Also, does anyone have any particular fondness for aftermarket trigger guards on ARs? The trigger guard on the XCR is comfortable for me and looks wide enough, but I haven't tried shooting it with winter gloves or anything yet. Are aftermarket triggers important to anyone or worthwhile to implement on the XCR?
No idea.

Frankly, I'd recommend skipping the AK lower thing. While it would be cool AF for XCR x39 owners here, it's probably not worth the effort. If it really was in high demand, Alex would probably already be doing it. As it is, most people just pick a different caliber than x39 to run in the XCR (and if .300 BLK ever really comes back online in steel case, there may be no real reason to try and run x39 in an XCR anyway since they are ballistically almost the same and the big advantage was cost/round).

I'd doubt you have much tolerance in terms of true position for the AR FCG. The fact that everything is centered on an AR FCG in the lower vs. off to the side in the XCR may be a bigger problem.

The trigger guard is going to be a personal preference thing. I personally like Magpul's....but their design is essentially just a narrow version of what's on the XCR already and there aren't any issues with heavy gloves and the XCR to begin with....

As for AR triggers to begin with, I can only speak for myself....but I would be unlikely to buy a lower (keep in mind, you'll be a manufacturer of a 'firearm' and that will mean a bunch of red tape with ATF to do this) just to get the option of a better trigger. The factory trigger is actually very good for a factory gun, only gets better with time and likely can be modified to shorten the travel to a point where it may be stellar. I think the market is limited b/c the gun is $2200 or so....retail price point will be a big factor. I'd think anything over $500 for a complete lower isn't going to sell well....and it may need to come with an expensive AR trigger to even command that much.

There will be warranty issues where a non-running gun may be blamed on your lower and you and RA end up in a finger pointing match. Not fun...but should be relatively easy to solve. If a gun works with the XCR lower and not yours, it's your baby.

You may be considering selling lowers that are completely machined but lack all the internal parts. That will likely be a tough sell and a nightmare to source XCR parts. I think you're going to have to sell a complete lower minus the stock (picatinny to AR interfaces will probably grow in time....).

I wouldn't waste my time with an 80% lower, but I can see the draw from an end user/owner's perspective.

Stocks may be an issue too. People love the ACR stock....with a picatinny mount, that's not an option so consumers are left with the option of a trigger they like or the stock they like, but not both. So...you may end up in the stock adapter market too. Not necessarily a bad thing as it's another growth opportunity for your company though sourcing the ACR stocks is getting harder.

Like I said, good luck with it. I think if I had your ability with access to 3D printing, it might be easier to redesign the XCR FCG to be whatever it is you're seeking (binary, etc.) than building a new lower.....but that's JMO. And....that path is also fraught with liability with ATF scrutiny plus the very fact that you're making a FCG in the first place.
 

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Discussion Starter · #11 ·
There will be warranty issues where a non-running gun may be blamed on your lower and you and RA end up in a finger pointing match. Not fun...but should be relatively easy to solve. If a gun works with the XCR lower and not yours, it's your baby.

You may be considering selling lowers that are completely machined but lack all the internal parts. That will likely be a tough sell and a nightmare to source XCR parts. I think you're going to have to sell a complete lower minus the stock (picatinny to AR interfaces will probably grow in time....).
We'll deal with the complicated issues with selling "firearms" later down the road. There are options (which include working with an FFL with a CNC machine, or getting an FFL form 7 for myself). I've thought a lot about it and I understand the paper work issues, but I figure my clean background will help me out when I do have to cross that bridge. Then again, for the time being I need to focus my brain power to getting this to work in the first place.

As long as I have the right materials, get my dimensions right and I have a machine that can cut things up to specification, there should be very little that can go wrong with making this lower. Making moving parts is a different issue all together. I still don't get how to make features on a small part that creates timings on a moving system like a firearm, so unless someone can design an XCR trigger, I won't be able to make one. A drop-in version of the XCR trigger group sounds interesting, but that's an idea for another time.

Materials for plastic 3D prints aren't up to the job for high-pressure rifle applications yet until metal comes along. The XCR in particular is a bad candidate for a plastic lower because it relies so much on the lower to help contain the recoil pressure, not to mention having only 2 connection points between the upper and the lower. A trigger isn't much better; bolt forces would snap or bend most printer filament materials.

Speaking of which, what kind of steel are the lower parts (FCG, bolt catch and mag release) made out of? I haven't heard about for the XCR specifically. I need to make a longer bolt hold open at some point.

I wouldn't waste my time with an 80% lower, but I can see the draw from an end user/owner's perspective.
That's very disappointing to hear from someone who understands/uses machine tools and is enough of a die-hard fan of this niche gun to visit its' dedicated forum nearly everyday. I was actually thinking about asking you if you wanted an 80% version of the lower after I got mine working. I know it's a bit of work, but I figured some people on this forum would be interested in making the XCR a ghost gun. I guess 80% lower customers are a niche market, and an 80% XCR lower would be a niche of a niche, so if you don't want one, what hope do I have for the rest of the market of 13K+ XCR owners (some of whom might not actually care or notice that it's not an AR-15)?
 

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We'll deal with the complicated issues with selling "firearms" later down the road. There are options (which include working with an FFL with a CNC machine, or getting an FFL form 7 for myself). I've thought a lot about it and I understand the paper work issues, but I figure my clean background will help me out when I do have to cross that bridge. Then again, for the time being I need to focus my brain power to getting this to work in the first place.

As long as I have the right materials, get my dimensions right and I have a machine that can cut things up to specification, there should be very little that can go wrong with making this lower. Making moving parts is a different issue all together. I still don't get how to make features on a small part that creates timings on a moving system like a firearm, so unless someone can design an XCR trigger, I won't be able to make one. A drop-in version of the XCR trigger group sounds interesting, but that's an idea for another time.

Materials for plastic 3D prints aren't up to the job for high-pressure rifle applications yet until metal comes along. The XCR in particular is a bad candidate for a plastic lower because it relies so much on the lower to help contain the recoil pressure, not to mention having only 2 connection points between the upper and the lower. A trigger isn't much better; bolt forces would snap or bend most printer filament materials.

Speaking of which, what kind of steel are the lower parts (FCG, bolt catch and mag release) made out of? I haven't heard about for the XCR specifically. I need to make a longer bolt hold open at some point.



That's very disappointing to hear from someone who understands/uses machine tools and is enough of a die-hard fan of this niche gun to visit its' dedicated forum nearly everyday. I was actually thinking about asking you if you wanted an 80% version of the lower after I got mine working. I know it's a bit of work, but I figured some people on this forum would be interested in making the XCR a ghost gun. I guess 80% lower customers are a niche market, and an 80% XCR lower would be a niche of a niche, so if you don't want one, what hope do I have for the rest of the market of 13K+ XCR owners (some of whom might not actually care or notice that it's not an AR-15)?
I wasn't saying to use 3D printed parts (certainly not out of plastic) for anything other than rapid prototyping, proof of concept and fitment/geometry (talking trigger or lower). You'd need a sintered metal printer for that (and the machines you could actually afford....that are still major $$$....aren't really yet up to the task).

Most of the better triggers use tool steel of different types. I doubt you'll get many manufacturers to tell you their material, or heat treat specifics as they are proprietary.

I think you underestimate how far up your ass ATF will be....but I'm sure you'll find that out as this progresses.

80% lower 'ghost guns' are a fantasy IMO. If you owned no other firearms, that might be a way around coming up on government radar. If you own anything NFA, you're already at the top of the list when confiscation comes a knocking. IMO, if it's time to bury your guns or hide them, it's time to use them....YMMV. Anything not NFA in most 'free states' isn't registered anyway, and if government demands registration, it's casus belli b/c it's a prelude to confiscation; nothing more.

And yes, 80% lowers are a niche market....to an already extremely niche market in the XCR.

My advice would be do this project because YOU want one....not b/c you think it's going to be a huge seller. If it turns out that way, then you'll be happier than a pig in shit. If it doesn't, you'll still be likely quite satisfied, a better machinist and proud of the accomplishment of having something one of a kind.
 

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Discussion Starter · #13 ·
I wasn't saying to use 3D printed parts (certainly not out of plastic) for anything other than rapid prototyping, proof of concept and fitment/geometry (talking trigger or lower). You'd need a sintered metal printer for that (and the machines you could actually afford....that are still major $$$....aren't really yet up to the task).
I get what you mean now. I'm just hoping my filaments will hold up after I print and test with them; I don't need any false negatives or positives from the plastic warping after the fact. One of these days I'd love a sintered metal printer or mold, but first the need must match the ambition.

Most of the better triggers use tool steel of different types. I doubt you'll get many manufacturers to tell you their material, or heat treat specifics as they are proprietary.
Think 4140 would work for the bolt catch? I got some of that lying around, not sure how I'm gonna harden it though if at all; not looking forward to grinding off the scales after oil hardening.

On that note I'm a little bummed that I have to make a new, longer bolt catch for this lower. Maybe I can get away with making a threaded extender that goes between the original bolt catch and the foot release? For that matter, I could look into making copies of the XCR bolt release and mag well parts if we want an alternative source for these parts.

Stocks may be an issue too. People love the ACR stock....with a picatinny mount, that's not an option so consumers are left with the option of a trigger they like or the stock they like, but not both. So...you may end up in the stock adapter market too. Not necessarily a bad thing as it's another growth opportunity for your company though sourcing the ACR stocks is getting harder.
I'm not looking to get into the stock adapter business, Lucky Irishman seems to have that covered (for as long as he can stomach it anyway). Haga Defense makes a 1913 stock adapter for the ACR stock, and Kinetic Development Group seems to have a steady supply of the ACR stock readily available for what ever rifle you need it for (there's probably more ACR stocks than ACR rifles that might need them). I'm also comfortable knowing that there is an adapter that connects a buffer tube to a picatinny rail out there. So with that, I feel like the 1913 rail is very versatile option to rely on. It's also relatively easy for me to design and machine, since I've made picatinny rails before.

My advice would be do this project because YOU want one....not b/c you think it's going to be a huge seller. If it turns out that way, then you'll be happier than a pig in shit. If it doesn't, you'll still be likely quite satisfied, a better machinist and proud of the accomplishment of having something one of a kind.
Sure, like I always say, if the toy you want doesn't exist, make it yourself. Still, I feel like this is something that at least a few people would want, and I want to be able to help with that. And while it would be nice to make a side business out of building metal parts out of my garage for a bit, it's not going to blow up with just one design. Leemajors and MountainMan's experiences with making parts for this community have really colored my expectations for this kind of work, so I just want to make what I design available to people who want them (I'm still looking for takers on that DP-12 rail I made, lol). That can mean partnering up with a Form 7 FFL or just making the mechanical drawings available for people to make their own, we'll see what happens.
 

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I get what you mean now. I'm just hoping my filaments will hold up after I print and test with them; I don't need any false negatives or positives from the plastic warping after the fact. One of these days I'd love a sintered metal printer or mold, but first the need must match the ambition.



Think 4140 would work for the bolt catch? I got some of that lying around, not sure how I'm gonna harden it though if at all; not looking forward to grinding off the scales after oil hardening.

On that note I'm a little bummed that I have to make a new, longer bolt catch for this lower. Maybe I can get away with making a threaded extender that goes between the original bolt catch and the foot release? For that matter, I could look into making copies of the XCR bolt release and mag well parts if we want an alternative source for these parts.



I'm not looking to get into the stock adapter business, Lucky Irishman seems to have that covered (for as long as he can stomach it anyway). Haga Defense makes a 1913 stock adapter for the ACR stock, and Kinetic Development Group seems to have a steady supply of the ACR stock readily available for what ever rifle you need it for (there's probably more ACR stocks than ACR rifles that might need them). I'm also comfortable knowing that there is an adapter that connects a buffer tube to a picatinny rail out there. So with that, I feel like the 1913 rail is very versatile option to rely on. It's also relatively easy for me to design and machine, since I've made picatinny rails before.



Sure, like I always say, if the toy you want doesn't exist, make it yourself. Still, I feel like this is something that at least a few people would want, and I want to be able to help with that. And while it would be nice to make a side business out of building metal parts out of my garage for a bit, it's not going to blow up with just one design. Leemajors and MountainMan's experiences with making parts for this community have really colored my expectations for this kind of work, so I just want to make what I design available to people who want them (I'm still looking for takers on that DP-12 rail I made, lol). That can mean partnering up with a Form 7 FFL or just making the mechanical drawings available for people to make their own, we'll see what happens.
Yeah, warping can definitely be an issue. No real way around that other than try it and see. The plastic parts would be just a rough idea anyway.

4140 can certainly be heat treated as hard as you want....question is: how ductile does the part need to be? Too much heat treat and it'll be brittle. And, since there's the possibility of a bolt slamming into it while cycling, that might be trial and error unless you have a hardness tester (recommended) for the factory part.

An extender sounds like a recipe for problems....JMO. Might work fine. Won't know until you try.

I know Haga and Kinetic were having trouble finding ACR stocks a while back....maybe that's changed. I was only briefly in the market for one when looking at building a SCAR.

I think LeeMajors and MM's experiences are spot on for this project. I think there will be interest, but it won't be a huge number unless the XCR gains massive popularity all of a sudden.
 

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Little off topic, but I just want to add that the stock XCR-L lower takes and feeds from 9mm MP5 mags with out any modifications.
Huh?
 

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Little off topic, but I just want to add that the stock XCR-L lower takes and feeds from 9mm MP5 mags with out any modifications.
Id like to see a picture of an mp5 mag being held in an xcr lower mag well without modifications….
I’m not saying it can’t, I’d just like to see it because it doesn’t have the same release mechanism. Not even close. Has someone been sniffing superglue? Or is there something we’re missing?
 
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