Designing XCR-L Lower Reciever with AR FCG

7561 Views 109 Replies 12 Participants Last post by Chowwow, 4 d ago

Chowwow

Discussion Starter · Oct 8, 2021 (Edited)

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, and 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.

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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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Chowwow

Discussion Starter · #101 · 3 mo ago

Quick update. I updated the CAD model to include angled chamfers. Doesn't look too different but I imagine it will be easier to mill. I'll have a 3D print ready soon, I hope it will be the last.

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I'll need to make tool paths on this model to teach a machine to cut the part. I'll probably need to make a jaw that will hold the Lower straight up and down to mill the 1913 rail and the space for the barrel bolt. It's hard finding machines that can mill objects that are 9.75" tall on the Z axis, everything else needs a little extra thought.

Below is the 7075 aluminum I bought last year for this project. Somewhere inside this canvas is my lower!

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It's a lot of metal and the lower is L-shaped, so I was hoping to arrange two lowers so I can have 2 out of one section. Unfortunately, it's not quite wide enough, so the second one is either going to be the Picatinny-XCR or even the AK-XCR w/ XCR FCG. I'll have to see what I can do about it.

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Chowwow

Discussion Starter · #102 · 2 mo ago

The print I made was red and printed with tree supports. It looks wicked in red and the tree style supports were easy to remove. There was some distortion in the print however, and that's no good when you are trying to check for accuracy in a part.

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I was hoping this would be the end of it, but when I tried to use my new Magpul MIAD grip, I found out that there still is a problem with using grips with backstraps/beavertails on them. I realized that the CAD model is accurate to the XCR-L, it's just that the design of the XCR itself is hostile to beavertail grips. Back to the drawing board; I'll keep it to test the new BHO location later.

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I realize that the issue with the grips is that the beaver tail section above the grip is too low, not that the radius was too tight. The latest design has less than .100" trimmed from the height of the beavertail section of the lower. I would have thought it was a lot of material gone but I barely notice it.

And while I was at it I found some space to drill a QD sling point into the lower itself. Based on what I've seen on how QD sling points are built into other guns and how Lucky Irish used to machine into his adapters, I decided to put the QD point on the spot on the lower where the sun don't shine. It's a little more than 1/2" away from the edge of the last picatinny rail, so it's as far away as I can place it to keep it from interfering with anything you might have mounted on the rail while still having enough material around it to stay strong and sturdy. It's less than 1/2" deep into a spot that used to be the thickest part of the aluminum, so it should not bother the internal operation points or weaken the structure. Machining the QD point should be fairly easy; I need to design a fixture to hold the lower at the appropriate angle to machine the hole and I need to find a small enough woodruff/t-slot cutter to make the inside slot.

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I'll get back in touch with my local FFL on a deal to use his machine, then see if what I've done works in the next print.

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Chowwow

Discussion Starter · #103 · 2 mo ago (Edited)

I have sent a couple of 3D Printer-equipped users the last files of the AR-XCR model for evaluation. I'll be eager to hear what everyone thinks about the design so far.

The last lower seemed to do the BHO arm well. This time there is a reassuring length of the BHO finger placed on the mag follower and no chance of it slipping off due to rocking.

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Bit of a gap around the BHO foot but just enough for clearance while keeping it in place.

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Seems to mount tight to the Upper. The edge on the magwell (in front of the mag catch) is a bit off and I'll move that a bit.

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There is still a bit of a gap between the receivers, so I'll move the front takedown pin another .010" down to close it up.

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Fitting and function checking the Mag catch and BHO arms.

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BHO head doing what it's supposed to do. Also checked cycling of rounds with snap caps and the rifle cycles smoothly and picks up rounds.

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Did a little work today and added indicators for the AR safety consistent with typical AR-15 models. The text is made so a 1/64" ball nose engraving tool can be used to CNC the words into the Lower. That's as small as a tool I can think of to use. I might do laser engraving instead to get text; I'm planning on laser engraving the serial number and roll marks anyway. I hope my choice of words on the safety selector are appropriate; they are also on the other side and written correctly. The separating nubs between the text are .100" tall and should not get in the way of the selector that you might be using.

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I also added back the radius at the mag well area because I liked how it looked and it seemed to blend in with the Upper better. Having a straight edge there would have left a curious shelf at the adjoining Upper.

Reduced the number of rear picatinny slots again to make the rear lighter and sleeker. Similar rifles like the Sig MCX have about this many rear slots and still manage to be versatile and be compatible with many stocks. I also made the angle of the rear cutoff more shallow so that the QD slot would not point at the user's hand as much and force the QD sling to rub into the shooter's hand. I hope it still looks good to you guys.

More waiting on this design until I decide to print another one. I hope that it's the last thing I need to do for designing!

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Chowwow

Discussion Starter · #104 · 2 mo ago (Edited)

I think I've officially done a ton of work on this project.

I've done some refinements of the design over the week. Among other things I've rounded out and evened out the lips of the mag well. The mag well is now equal in width on both sides and the BHO foot is centered with the mag well. The front of the mag well lip protrudes less and is now rounded at the corners to make it less likely to snag.

The both sides of the mag well lip have a radius so that they match each other and feel nicer to handle when reloading or gripping.

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The safety labels have been replaced with engravings of bullets like they are on the XCR, making them smaller and easier to engrave than letters. They are to be engraved with a 1/64" ballnose endmill or laser engraved.

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It was brought to my attention that an aluminum milled QD slot may not be the strongest solution for a QD attachment point. I looked around and Grovtec makes an externally threaded steel QD sling mount cup that I can screw into the aluminum lower. With rockset or loctite thread locker compound, I think screwing in the QD mount cup will be the simplest and strongest solution.

Here is what the cup looks like after install.

Work continues and I'll need to head to designing the tool paths for milling the lower. I realize now that milling the holes at the back of the receiver for the recoil spring and bolt carrier buffer will be difficult with the rest of the lower being in the way. My idea is to get a foot-long endmill to mount to the CNC spindle and have that end mill make shallow cuts to make those holes. The 3D printer will be handy to print fixtures and jigs to lock down the metal and rotate the part over a 3-axis endmill. I'm going to need at least 6 sets of soft jaws to assist in 6 operations on the CNC. This'll be the most complex thing I'll have built ever on any machine.

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Chowwow

Discussion Starter · #105 · 2 mo ago

I'm calling it, I think I'm done designing. Thanks to some fellow CAD designers I met on here and on Reddit, I was able to cosmetically refine the design to something I'd be proud to handle. Here it is with a Magpul MIAD Gen 1.1 grip and a QD sling. The gap in the pistol grip area is mostly eliminated and should seal out when you screw in the grip, I'll back off the beaver tail a bit more to give it some more clearance just in case. The QD sling location looks nice and doesn't get in the way of your hand if you fold it away the right way. Also doesn't get in the way of something like the ACR stock if you have it on there. In the end, I wasn't able to add trigger guard customization to the AR-XCR, but it turns out the QD slot was a very nice touch that I never realized was so useful.

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Gap between receivers is mostly gone, I attribute any more little gaps as due to plastic tolerances in the 3D print. Lines consistently line up nicely with my new model upper, so I'm confident it will blend in with other newer model uppers. The lower continues to pass all function checks as well, leading me to be confident that we have a working lower. I believe any other inconsistencies or issues would only appear after machining the lower out of metal; once we are manufacturing with aluminum tolerances I'll be able to see if I need to update the design anymore beyond this.

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I think I've pushed the design as far as it can go. Now is the time to sit down and start making those tool paths. I forgot how involved CAM was and it's going to require all my concentration to get the tool paths programmed. As such, I'm suspending progress on the AK-XCR and other projects until I've programmed the cutting paths for the AR-XCR.

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Time to start getting this out into the world!

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Sean K.

· #106 · 2 mo ago

Congrats! It looks great. Very impressive the amount of knowledge and effort you've put into this.....I wish you the best with it!

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tommyid1

· #107 · 1 mo ago

This is pretty awesome!

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Chowwow

Discussion Starter · #108 · 1 mo ago

tommyid1 said:
This is pretty awesome!

You're pretty awesome!

Stay tuned and buy one when they are available!

Bonus pics for the current AR-XCR, all the parts screwed in for good measure. All parts seem to fit really well. I used the ambi mag release this time and noticed some resistance with moving it from the left side. I'm going to loosen the tolerances around the mag catch slot in that area so that it slides more easily.

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Good fits otherwise.

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The not often seen left side of the lower. It seems to match well to me.

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I'm in talks with someone to secure a CNC machine for myself in the last few weeks; it's expensive but the deal is looking pretty good so far. I'm still planning to work with an FFL to have these produced for public sale.

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Sean K.

· #109 · 1 mo ago

Congrats! It's looking really good. Good luck with the CNC purchase....got to be exciting!

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Chowwow

Discussion Starter · #110 · 4 d ago

Some idle thoughts while I'm working on some major things:

An assembly of an XCR-L using parts I've gathered over the last few months. Looks like a monstrosity doesn't it?

Like I mentioned before in the rear adapters thread, the number of picatinny slots available in the AR-XCR may not be totally necessary and in fact seems to break up the look of the rifle. I suppose it's not a huge deal but I'm not sure if anyone is going to use a stock mounted as high as it can currently go.

A reminder as to how the older design of the AR-XCR looked with the sloped top rail. I may want to try again with something like this.

Big things have happened but I'm not going to waste people's time with small updates until I can announce something substantial. The biggest challenge now is relearning how to do toolpaths on my CAM software, since I've been out of practice for awhile. Should be up to something big in a few months though.

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What lower should head to manufacture first?

Picatinny-XCR

AR-XCR

Mag Well-Swap AR-XCR

Designing XCR-L Lower Reciever with AR FCG

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