Designing XCR-L Lower Reciever with AR FCG

Here's the latest version of the lower in CAD. I've added the clearance for the Upper's bridge support and widened the space where the RA rollmark goes on the Lower; the latter has resolved the issue where the space for the mag release arm was disproportionally short.


Of particular note is that I've shifted the entire FCG .050" forward; this seemed to resolve the issue of the safety detent being too far forward (the error suggested that the safety hole was too far back) and kept the hammer pin hole centered on the front lobe. This means that the hammer now .050" forward, but it may have needed that distance to travel far enough to strike the firing pin.

It looks like at this stage, work for this lower will involve a lot of guessing and checking. It's a good thing plastic is cheap. I have to be sure to adjust my print settings to make a more durable fit test.

Seeing Jordan636's XCR with the Zhukov stock has led me to believe that some people might need the slightly longer rear picatinny rail that the Lucky Irishman adapter provides. I may try the alternative rail design when the next print gets made and see how that handles.

Small update, I've expanded the rear rail to match more like Jordan's old rifle. Should be helpful for people who need the extra length. I don't think it looks too bad either, it's a compromise between the really long rail I had in the beginning and the shortened rail that didn't appeal to me quite as much.




While I was at it, I made a model of the bolt hold open too. Its tiny details might be off but the main details like the length and screw thread location are caliper-accurate. I think it's good enough to make a replica part if necessary.

Making a model of the BHO might be necessary because when we extend the lower to fit the AR-15 FCG, we might need a longer version of the BHO to take its place. I will try to use the original BHO and use a longer screw or plastic extender to make up the quarter inch length difference before I do that however. I think doing that will still make the BHO sturdy enough for the job.





At this point I need to print another one out and do more guessing and checking. But I think I really got it close this time: cosmetically it's mostly there (and all of the production lowers I've seen seem to have some variance to them anyway) and it's only a matter of getting these hole locations right before we have a functioning receiver. The magwell was a little too tight for magazines in the first print, but it could have been due to plastic deformation that was the cause.

Recent events with AWB 2022 being passed narrowly in the US House has been a cause for concern for me, since it (in Section 2, item (40), sub-item (M)) prohibits the manufacture of receivers like this. While whether it can be passed as law is another kind of discussion, it does hold my feet to the fire to getting this thing built. At this rate, I can have both designs finalized by September and something built in metal by October or November. If I don't get a CNC machine for myself by then, there are several FFL machine shops around the nation that I contract to mill this thing out; even if it costs me $600 for one of these it'll be worth it. That should beat out legislation in the chance AWB '22 gets ratified by the end of this year. Being able to produce this thing in numbers for other users, however, gets less predictable even if I can file for an FFL form 7 in time. Even if the window for major production gets slammed shut, the fact that this data exists keeps hope alive. Either way, I need to pick it up a bit to make my target goal; August came by really quick this year, I swore February was last month...

Candiacan said:

Either way, I am blown away by your talent and creativity.

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I wish I went to school as a mechanical engineer, I'd be able to build some serious shit I'll tell you. As is, this is just pure research and nose-to-the-stone measuring.

I've spent the last 3 months (ah shit) assembling my new 3D printer to speed up the prototyping process and check the precision of my model. That, along with the expanded hours I've been working at my job have left me little time to fraternize on this site or work on the CAD. I've spent nearly $1000 on top of the printer on accessories and parts to make the printer more reliable and sturdy and it still needs some TLC. Not even 2 weeks ago I wanted to tear the hairs off my sack with how frustrated this monstrosity made me, but then I managed to correct a few things that led to another. As of today I finally have the second 3D print of my lower and the first fitment prototype.


There is some warping at the magwell, it's the first part of the lower that is actually printed, but it's intact enough for us to get an idea on how it looks like. I'll still have to make adjustments with the printing process so we don't get this kind of warping but the important part is that the main body of the lower is to spec.



The new lower mated to an XCR-L standard upper, as provided by Kermit. Much of the support material can be pulled off but the main mass at the bottom of the lower is gonna take more manual work to reveal the trigger area and grip. A lot of the lines I designed match up with the upper and the mating edges between the upper and lower sit flush with each other.



The main issue is that the model has about an 1/8" gap at the rear mating surfaces of the receivers. The lines at the side cheeks of the lower also don't match. These are being corrected on the next CAD model revision, but this goes to show how important 3D printing is with this process; the model has been this off since the very beginning and none of my notes caught this.



Comparison between the old print and the new one. Notice that the hole locations in the new model have been moved over a bit. The new print is higher quality and shouldn't crack apart when I install the FCG. The mag release slot also seems to match with the slot on the magazine.



Comparison between my XCR and my would-be pistol/SBR build with the printed lower. The front takedown pin is lined up perfectly. The point where the rear of the lower angles up to the radius is also perfectly placed despite the gap at the end. The angle of the magazine isn't quite right and that concerns me. However, the last pic of the lower with the mag seems to have the top of the mag lined up correctly with the lower, so the bolt should be able to strip a round out of the mag without issue. I also noticed that the curvature of the front lip of the magwell isn't as pronounced as that on the original rifle, but that can also be an artifact from the forging process and may not be important to my lower.

Once I have the lower freed of the support material, I'll install my trigger into it and do my best function check with it. I should still be able to use snap caps to test the bolt cycling over the magazine. After I tighten these dimensions, I'll make a third print and use it to do a complete install and function check; it may not be sturdy enough to shoot the rifle with but it's certainly durable enough to dry-fire and cycle rounds with. Currently, the Pic-XCR lower is an arbitrary 10" long, but the presence of the gap in the receivers I saw today has made me consider shortening the entire lower by the length of that gap, just to keep the lower as small a possible. I might round out the length to an even, whole number just to sooth my OCD.

As a bonus, here is an idea what the final expanded lower for the AR FCG will look like:


It looks like the same damn thing, but calculations show the AR-XCR lower could be 10-15% more material than the Pic-XCR lower. I may have to do something creative with the lobe that covers the hammer pin on the OG lower; the AR FCG doesn't have pins that are that long. I also have to decide to go with either .25" or the .29" I calculated on the additional depth for the lower; I can't tell which works best until I print it. Otherwise, straightforward work.

In other news, the CNC machine I was planning to use turned out to be inadequate for my uses upon further research, so I canceled my order for it. With 2022 nearly over and my weekly schedule now very different than it was earlier in the year, I've shifted plans to have the CAD work for both types of lowers done and function-testing 3D prints for both ready by the middle of December. On the upside, there are FFL CNC machine shops in other parts of the country that I can call on to have these made. ZYCI in Georgia and Versatool & Die Machining and Engineering in Wisconsin are two places I've found that might be able to machine these for me. I figure it's more than a regular lower from Robinson, but I'll also bet good money they'll deliver a lot faster. Once I've confirmed that the CAD models are producing functional parts, I can ask one of these guys to produce one for me. And if it works really well, I'd be willing to provide the CAD data for other users to send to one of these guys to have these lowers made and transferred to you. I'll leave a tip jar or something out so people can help support me for this work.

Update: FCG holes and grip screw hole appear to be in position and trigger seems to be working well. Beaver tail on the lower protrudes too far out and any pistol grip with a beaver tail will not fit. BHO appears to be too far forward and BHO foot won't screw in. Bottom slot for trigger seems to be too far forward and slot currently rubs against the back of the trigger. Magazine catch is too low on the receiver by about .100", so magazines are not quite high enough for the bolt to strip off rounds from the top of the magazine.



I finally liberated the second print from the supports and got to work. The XCR FCG seems to be in just the right place and strikes the firing pin and resets upon bolt pull back. In this photo you can see how the mag catch space seems a little too low to catch the receptacle on the magazine (placed here at the height where the magazine would normally go on a default lower). When the mag is at the proper height, it eliminates the shallow angle of the mag problem I was seeing before this.



Had to force the magazine to reach this height. The space for the mag catch was out of spec and a bit too narrow than expected. Still much too low to work.



I might want to bring the slot for the trigger backwards by .50". I can't really tell how the trigger pull is but it feels stiff to pull the trigger like this. The slot for the BHO foot is actually up to spec, it just seems like the hole for the BHO is too far forward for its good.



The rear of the lower protrudes too far out for any pistol grip with a beaver tail to fit on the lower. The placement of the grip attachment itself, including the safety detent hole and grip screw hole are on point, however. I wonder if I should thread the hole with a fine thread like default AR grips or stick to the coarse thread that the XCR uses. If it's being screwed into aluminum, I would like to use the coarse thread screw.



Trigger clicks and resets properly. It's just that the magazine is sitting a bit too low for the bolt to strip a round properly.

Bonus:




I bought a Timney Impact drop-in trigger for this project and used it in the mean time to check the default XCR lower. The placement of the FCG holes is precisely like that of the AR-15 FCG, just on a lower profile position. Without that .25-.29" height, the hammer leans further forward than normal and the hammer is so long that it gets caught under the bolt on pull back if you tried to use this trigger on the XCR. This strongly indicates that when I raise the height of the AR-XCR lower, I won't need to move the positions of the holes anywhere to correctly place the hammer.

I've already made corrections on the CAD model. The lower is now 9.75" long instead of 10". I'll sleep on the changes before I decide to hit print on the next one. On the next print, I'll focus on printing the mag catch space accurately. I'm concerned the radius at the mating corner between the receivers might be off but we'll just have to see how the print turns out.

As we get closer to being done I need to think about how this is going to be machined. Right now, the lower emulates the OG lower almost exactly. However, the OG lower is forged (where a piece of heated aluminum is sandwiched between two dies that have the shape of the intended part), which is why it has a seam running along the middle of it on places that have not been machined off and why it has so many rounded and embossed edges. A CNC machined part can also have this many embossed edges but it would take more time and thus more money to make. I was thinking about making the edges square and straight, like that of the CZ Scorpion Evo or the Mac n Cheese 3D printed lower:



It'll take some time to make a mock up, but I can show an updated version later, if people like this idea.

Speaking of CNC machining, our own @SheffieldTactical is an FFL CNC machinist. If I don't get my own machine by then, I'll work with him to simplify the design and have his equipment cut out these lowers. Programming tool paths is just as involved as designing in CAD, and since I know what equipment he's using, I should be able to program the tool paths with my software at my place, which should cut down on the set-up time and labor.

SheffieldTactical said:

I’m in like Flin!

Click to expand...

I'll let you know when it's ready.



This is the first time we've had an intact lower that's completed the receiver assembly. Did you know that the mag catch on the XCR is pivoted up along the horizontal by a small degree? That would explain the bit of wear at the tip of the mag catch but not at the rest (like mine!). With that realization, the magazine now seems to lock at the correct angle and height without having to elevate the screw hole for the mag catch. I remembered the bolt was able to strip and cycle snap caps, but after I tried doing it again recently, the caps got hung up on the upper. The angle of the mag catch might need adjustment, but I think it's more likely that the plastic lower was deformed and bent after I tried to cycle the BCG over the trigger group.



After installing the trigger group, it was found that the hammer doesn't fold back as low, so it binds with the bolt and stops it from pulling back and going into battery. At first I thought the pin locations for the trigger group were off, but now I think it's because the trigger well is misplaced. It also turns out I was not using mil-spec for the location of the slot hole for the trigger; AR mil-spec sets the trigger hole a lot more back. I'll see on the next print if it makes the trigger feel better.


Timney Impact trigger here to help locate the hole locations. What's happening is that the trigger is being forced back by the front wall of the trigger well.


You can see the gouge where the hammer sear bit into the front wall of the trigger well as it was swinging back. I've updated the CAD to move the trigger well .035" forward, and that matches the OEM lower, but I can only hope that's enough.


It really rustles my jimmies how I can't get the rear lines of the lower to line up with the upper. Still, it seems like different models of upper have different locations for the rear lines: the lines are too low for the Standard length upper, but then they are not wide enough for the Competition length upper (indicated in red lines). I'm now a bit torn on where to put these lines, as the OEM lower has some slop with their locations too. I may try to match them with the Competition length upper for compatibility with newer uppers, or just stick to emulating the OEM lower.



Pistol grip almost fits, but still about .050" off. The grip dimensions are mil-spec but the beaver tail over the grip is still in the way. I might want to cut down on that radius to finally get a pistol grip with beaver tail working.

Even though the XCR-Pic lower isn't quite there yet, I think I'm about the point where I can actually start developing the AR-XCR lower. We already know that the trigger group pin holes are on point, the mating surfaces between the receivers line up nicely, BHO location seems to install correctly, and that the pistol grip attachment can still be refined as I go along. The AR-XCR lower will have an extended magwell, so I'll probably follow mil-spec on the location of the mag catch rather than following the discovered XCR measurements due to increased chance of binding. I'm starting to run out of filament, so I don't know if I want to jump right into printing the AR-XCR lower or make more refinements on the XCR-Pic lower. The XCR-Pic lower is to be the grandmother of all future XCR lower designs, but at the same time, I didn't really intend to have this version of the lower available for sale; it's just a reference design. Of course, it's also important to get the progenitor design right so that it's daughters are easier to work on and function right out of the gate too.

Here is the latest printed lower and I had to blend a white filament from a different brand when I ran out of the blue. Unfortunately, the while material is too brittle to do function testing, so I had to focus on checking the latest changes I made on the model.

I've finally noticed that the angle on the magwell is steeper than the regular lower that cannot be explained as due to plastic warping; this was due to the measurements I made with my angle finder previously. I've done some trigonometry and adjusted the angle of the magwell to something more like the original, but FYI, it's not the same angle detailed in the mil-spec of the AR-15.



I'm going to go off-script with the remaining adjustments. The rear lines are back in line with my recent-model Competition upper. The grip arch is still off and I'm going to cut it off significantly to ensure compatibility with all grips with beaver tails. Trigger hole slot seems to be on point this time, and the trigger sear doesn't bite into the wall of the trigger well anymore, so the trigger release is as crisp as the regular lower. Still, there is not very much clearance for the front of the trigger sear (not even a paper width); I've determined that I should move the trigger group .020" back towards the rear so there is more clearance for the trigger group from the front of the well. I wish I had X-ray vision, because doing this would move the hammer face a bit away from the firing pin, but I don't think it would be enough to cause any problems.



The BHO seems a bit too close to the front of the lower, and the foot snags with the magwell. While the BHO is positioned to catch the mag follower well enough, I need to make adjustments to the position of the hole to make the BHO work on this lower.



In other news, the picatinny rail on the lower doesn't add too much to the LOP. So if you had this 1913 railed lower, you can install an ACR brace/stock to it using the 1913 Rail ACR Adapter from Haga Defense. Such a combo should not be much longer than my current set up of the Lucky Irishman ACR folding stock adapter attached to the default lower.



The Pic-XCR lower is still being fine-tuned but the general dimensions are pretty set in stone. To give you a better idea of the dimensions of the mother of all lower designs, I've included a pic showing the general dimensions and thicknesses picture of the lower. In short, my lower is 9.75" long, about 4.65" tall, the distance from the mating surface of the upper to the 1913 rail is about 1.2", the thickness of the metal at the "auto sear" section is .16" and according to calculations, it weighs 13.705 oz (.8565 lbs). If you have issues with these dimensions, let me know.




Finally, I went ahead and made the AR-XCR lower in CAD. The pictures speak for themselves. Because the AR-XCR lower is taller than the default and the Pic-XCR lower, the rear 1913 rail can have one more slot. The two pictures below show one lower with 6 slots and another version with 5 slots and a sharper cutoff. Is more rail space better, or is there more than enough in this case?


The bottom of the AR-XCR lower with the original BHO shown below. The BHO is definitely too short now, but not that much. I'm pretty confident that instead of making a longer BHO, a longer screw and plastic extender should be enough to compensate.


General dimensions of the AR-XCR lower. It's mostly the same as the Pic-XCR lower but taller by .29", and the "auto sear" rear section is about .25" thick. According to calculations, the 7075 aluminum lower should be 15.566 oz (.9728 lbs), so under 2 oz heavier than the Pic-XCR.



More work ahead, hope to have a prototype for function testing ready by the end of the year!

AEnemaBay said:

Hell yeah, brotha. If you want help beta testing the lower (3d printed version) I could whip one up and test on my XCR as well. More data points, mo' betta.

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I'm glad to see you are around and that you took the time to read this thread! Do you have a 3D printer? I'd much rather send you the data files to print your own and have you check it out yourself than to send you one. Now that the project has progressed to fitting well enough to function, the printed lower can be interpreted as a firearm and I don't have an FFL (yet). Though I certainly would like direction on the features on this system that would appeal to the most number of people.

AEnemaBay said:

Looking back, you mentioned wanting to try to make one that accepts AK mags. Maybe a solution like the MGI Hydra that can swap magwells would work? That's something I've been meaning to try and work out for a long time, but other projects got in the way.

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I'm glad you pointed me in the direction of the MGI Hydra; a feature like that is certainly something I can do. The question this now raises is does implementing this feature warrant replacing the public launch of the AR-XCR lower to one that offers swapable magwells? I want to release something that is versatile and offers as many features as the design can stomach while having the least amount of dependence on me to make proprietary parts. I'll set up a poll on a separate thread on what to do with the future of the lower project.

After studying Abran007's lower design, making the AK-XCR lower has been something I've been interested in doing but I realize that in addition to designing the lower, I would need to provide a pin and spring as well as design a proprietary mag latch. Still, a mag well swap feature is an idea I'm intrigued with, because this allows for smaller parts = smaller metal stock = less scrap = more affordable parts. Also allows for the installation of aftermarket AR trigger guards too. Though I will tell you that my aim is to make the magwell and FCG sections very tightly joined; no messing around with hitch pins here, tight tolerances, captive pins and rubber mallets are the order of the day. The following is a basic idea on how I'd do this:

Flskunkape said:

If you made an ak mag accepting lower I would buy one! I miss shooting x39 and have a fair bit of ammo but can't talk myself into doing a 2k gooched out ak with zenit hardware.. and i hated the x39 stanag magazines. Anyhow if you decide to go the route of replacement front maxwells I would be very interested. Thank you for taking on this project, it's been very interesting watching the process.

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Glad there is interest. I know abran007 designed a lower for AK mags, but the problem is that when I asked, he said he didn't record or remember any of the dimensions and thus I'm starting from scratch. It's why I've made it my mission to save this data on to code, so that these designs survive whatever may happen. I'll go over my thoughts and the challenges regarding an AK-XCR lower in another thread.

More work ahead on the Picatinny-XCR lower. I'm dialing in the dimensions of the base design .010 of an inch at a time. Again, I really want to get the dimensions right on this version of the lower, so that anything else I make will work just as smooth.


Trigger pull is crisp and the hammer doesn't bite into the walls of the trigger well. The main issue I've been having is getting the BHO right; the foot fits poorly while having a gap in front of it. The finger of the BHO is close to losing contact with the follower of the magazine though, so instead of moving the hole for the BHO around, I've decided to move the expanded areas around the magwell back a bit to even out the opening space for the BHO foot. That way the BHO can function while leaving enough space for the BHO foot to move. It's been an intermittent problem for some time, but now I think it was because of the spacing of the magwell swelling before the BHO space that was causing the issue and not the location of the hole for the BHO.



Pistol grips fit flush now that I shrunk the beaver tail considerably. I'm thinking I cut off a bit too much, so I'll be adding back some material that will bring the beaver tail a bit further back.



Snap caps load and eject like they would with real rounds on the regular lower. Got to be real careful about the dimensions of the lower so the rounds cycle as they should.



A small (.010") gap near the front takedown pin appears often, while the rest of the lower is flush with the upper. I previously thought it was due to bending of the previous plastic lowers, but now I think the location of the front pin hole is too high up, causing to upper to mount higher up on the lower than it's supposed to. This has contributed to cycling issues I've seen in older prototypes, where the snap cap would strip off the mag and travel at such an angle that would miss the chamber entirely and jam in the receiver. I've lowered the front takedown hole by the distance I measured at this gap that should make it flush. Oh, and the fact that the line over the magwell swelling doesn't line up with the Competition upper bothers me. I think it's at a small angle with the vertical on the default lower, which is why it's so off when I draw a perfectly straight line on my design. What the hell is with RA sneaking small, non-standard angles on functional parts of the rifle like this? No designer working a 9-5 job would use these numbers for these angles at locations like this!



I've already made the changes to the CAD but I really should be working on this lower slowly to ensure it all works. The exciting rate of filament use is starting to get to me though; this black prototype is the 5th lower I've made, not including 2 partially complete prints that were made when I ran out of plastic.

I'm already printing up the first AR-XCR lower based on the latest changes. I hope to test my refinements I made here on the lower that we've been waiting for all along. I've decided to keep things simple and to have the AR-XCR fully developed and in production before I explore other features and ideas. I'm still concerned about what to do with the BHO for that one; I'd really prefer not having to build a new BHO arm out of metal as small parts require more precision and care to make. Here's hoping I can get it to work with the existing BHO and a longer screw. At this rate, my plan is to have all CAD work and prototyping done by the end of the month (it's the fine details that need work) and to have something built in metal by May or June, if not earlier. I greatly appreciate more feedback as it comes.

The AR-XCR lower fits well with the upper and the rear lines blend well. The lines along the magwell are off now and will need adjustment. There is still a bit of a gap between the upper and lower at the front takedown pin, so I might need to move the pin again. There is plenty of space in the receiver assembly for the hammer to travel without snagging on anything and the trigger feels crisp and light. The screw hole for the pistol grip got shifted in the CAD when I expanded the lower, so no pistol grip this time.



As a model, I used Elftmann Tactical anti-rotation pins to show I had enough clearance around the pin area to use them.



The issue I think I'm having is that the flat face of the hammer of the Timney trigger I'm using doesn't go far enough to hit the firing pin like the normal XCR hammer does. When I pull the trigger, I see the bolt carrier vibrate when the hammer comes down, but it doesn't make a solid ping what I can interpret as a strike on the firing pin. Here it is, the Timney trigger hammer vs the XCR hammer in the background; you can see that the XCR hammer travels more forward. I put a drop of paint on the hammer face of the Timney and tried pulling the trigger with the rile assembled. When I checked the firing pin afterwards, I saw that the paint had splashed around the pin but there was no paint spot on the pin itself, so that indicates there is little to no hammer contact.



I now face a dilemma: move the trigger group forward a significant distance to get the hammer to land on the firing pin, or lengthen the trigger well so that the hammer has more space to lean forward and hit the pin. I might have to do a little of both, though how much distance is enough is difficult to figure out without a way to measure it. The trigger seems to be in a perfectly fine spot, so we should not move the FCG too much or we lose the alignment.The best I may have to do is come up with an arrangement to where the AR hammer reaches roughly the same position as the XCR hammer using visual comparison. This could be a long road.

AEnemaBay said:

Sorry for the delay. I didn't get an alert that there were new posts. But yes, I have a 3D printer. It's looking really good.

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I'll PM you the updated STL when I'm done with it. As it is, it doesn't work and I need to rework what I'm doing here.

roflcopters said:

No, I mean it IS supposed to hit at 90 degrees. I was thinking maybe moving it forward a little, or angling the pin holes a tiny amount (tilting forward), could resolve the issue. Might just have to test different hammers, or design a replacement hammer that sticks further forward.

Click to expand...

I'm trying not to go crazy with making custom parts to get this system to work. Certain changes (like a new BCG or firing pin) can potentially really affect the reliability of the rifle which will really piss off the old guard; hell it was hard enough to get the magazine to feed rounds reliably based on reverse engineering alone. I was stressed out enough at having to make a custom BHO or mag latch. I don't have the resources to make multiple small parts and I'd like to avoid it if possible.

For the next iteration of the lower I am imagining a layout similar to that of the AR-15. The trigger guard and trigger well are going to be moved much closer to the magazine well, and I'm going to have to use the AR-15's mag catch and bolt release. This is because everything around the FCG, trigger guard and pistol grip are locationally dependent on one another; so if we have to move the hammer forward, everything thing behind it sans the rear Picatinny rail has to move with it. \While this lets us use standard AR-15 mag releases, we will be losing the XCR's ambi BHO unless I can figure out a system that works in such little room with the AR mag catch arm in the way. As of now it's the most reliable way to implement the feature. It also allows me space to place holes for custom trigger guards. What I want to know is if exchanging the BHO style is acceptable to the potential buyers of the lower.

AR-15:



VS current AR-XCR:




I've brainstormed some ideas for BHO on the AR-XCR, and here's a list of those ideas:
A. AR Bolt release: Use standard bolt releases for the AR. Leaves room for aftermarket triggers guards.
B. Shortened BHO where the foot ends at the high end of the trigger guard. Also leaves room for aftermarket trigger guards.
C. Exposed BHO, where the foot ends roughly at the low end of the trigger guard but the bottom portion of the BHO arm is exposed by the trigger guard, so you can see the BHO move up and down from the front of the trigger guard.