Designing XCR-L Lower Reciever with AR FCG

Sean K. said:

While I have been on the fence about the whole thing, this would be a deal breaker for me....but honestly, I'm not your target audience either. While I am a trigger snob, I don't see a big enough gain going from the factory trigger to an AR one to justify losing other features that make the XCR ergos better than anything else out there.

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You prove to be a difficult man to please, Sean. Seriously though, what do you think of an exposed BHO arm along the front trigger guard? I swear there is at least one gun out there that is kinda like that. All parts behind the hammer are tied together; I'm working on a promising idea but I can't change physics at some point.

Another thing to consider....depending on how this brace thing shakes out....it could affect the market for the lower if folks get locked into NFA lowers.

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What do you mean by NFA lowers? You mean people being forced to register XCR pistols as SBRs and needing to laser engrave info on their lowers? It is what it is to me, though I was expecting that the built-in 1913 rail would replace the need for a pistol cap and that the pistol can be run supportless out of the box.

Flskunkape said:

I would be cool with an AK-XCR lower using ak mags and factory trigger, just being able to use the mags God intended that round to be fired from would make a happy camper here.

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I suspect the AK-XCR with the AR FCG would be easier to work with since there is less stuff in the way... though I will only know for sure once I get to work on it. It's possible that the needed magazine latch will have to be mounted steep and no existing mag latch from another rifle would work in my design.

Sean K. said:

Just being completely honest about my personal opinion....no one else is required to agree.

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Never the less, I am inclined to provide! I'm gonna run a few experiments that sound promising. The shift to a standard mag catch arm has been a blessing in disguise; though virtually guarantees that I have to make a new BHO. I'll tell you, this situation with the AR FCG has been the most dire one since August; it might even side track the whole project. Though I did warn myself that this could be the case when the project first started, I didn't think I had so narrow margins in which to fix it. But that's the great thing about CAD; it simulates reality before you invest big bucks on what you're doing.

What I meant was....once someone NFAs a lower, he's less likely to want to buy another lower for that same gun and have to pay another stamp fee and go through all that again to get a different trigger.

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I see now, that's a very interesting situation. The NFA situation really affects those who wanted to install the binary trigger on their former pistols, though I'd still argue that the lower is designed so one can still run the pistol without a brace or stock and go their merry way (I'm still trying to understand the ruling, but non-stabilized pistols are still non-NFA, right?). My primary vision was using competition triggers on 16" barrel rifles in specialized circumstances, though I did intend to use the binary in a pistol/SBR with these. In any case, I have no say on how people use these; just that I am able to deliver on expectations. It's taking longer than I expected, to the point that the speed of legislation and executive orders are becoming a threat, but I am doing this on my free time after all, lol.

So I may have overreacted over the last few days over what I saw with the Timney trigger.

I installed the binary trigger last night and the trigger seems to be working as intended. Unlike the Timney, the Frankin trigger's hammer is more similar to a mil-spec AR hammer, and that makes a big difference. It looks like the current design of the upper will work fine as long as your trigger's hammer is shaped like a mil-spec trigger.


The resting angle of the hammer is now similar to that of the regular XCR hammer.



Hammer faces line up when super imposed like this.



Like the regular XCR hammer, the Binary hammer rubs against the firing pin when sliding the receivers together like this. This indicates that there is more than enough contact between the hammer and firing pin even while resting.


My new Binary trigger has fresh marks on the hammer face after a few dry fires of the -L. This is a good indication that the hammer is making contact with the firing pin when pulling the trigger. Binary function appears to be working as well, on dry fire at least.



There is a solid and metallic ping when pulling the trigger, so it's likely that the hammer is hitting just right. I was concerned before about the lower's design not working because changing it would have unraveled the structure of the design. But now it seems that because the Timney trigger had a taper below the hammer face and not a corner like that of the Franklin or a mil-spec hammer, it hung back a little more at rest than coming down like the other triggers could do.



My plan now is to tweak the design some more and provide a little more space at the front of the trigger well so that tapered triggers like the Timney have space to fall forward a bit more in order to reach the firing pin. Once I'm satisfied with a print, I'd willing to send the STL files to those who'd like a preview version to print out and try. Though it seems to work fine for mil-spec, I'd like this lower to be compatible with all AR triggers and it looks like I need just a bit more empty space to provide some clearance. Overall, this has been good news and the design can go on mostly as planned; though I wish there was a way to test if the lower works exactly as intended in a live fire test. My lowers are strong enough though, so I could go out and test at least two or three rounds through them using live ammo at the range.

I feel like I am ALMOST done with the design now. I'll print up a full prototype after I get a few details down, but functionally it should work.

Below is the current design of the lower. All the lines match that of my competition length upper; they don't match that of my older model standard-length upper nor can I guarantee that they fit with the other shorter length uppers. By my estimation however, the competition length upper is the most recent and should match that of new model uppers. The current design has 5 picatinny slots on the rear; it's shorter than the last design but still longer than most rear 1913 sections. You may not notice this, but I also moved the trigger guard around so that the dimensions are more consistent and logical, and appear to be a little more like the standard trigger guard.



The roll mark section of the left side has a radius that matches the corner radius of the adjoining upper.


Here is what the rear section looks compared to the standard lower, just a bit longer than standard. Again, the AR-XCR lower is 9.75" long. I'm considering on making the next revision shorter to save on weight and reduce LOP, but I don't see it making that much of a difference and I don't want to compromise on the structural integrity of the lower.



Height comparison between standard lower and AR-XCR lower. Honestly, for all the effort it took to make the XCR as small and low-profile as possible, there is not much difference in bulk and weight between my lower and the standard lower and in exchange, we as consumers were forced to buy or seek out proprietary parts that are harder to get. Ounces become pounds in the field though, so I guess shaving those ounces off in exchange for proprietary parts was worth it?



I printed a section cut of the AR-XCR lower to see how the trigger interacts with the BCG in the upper. I thinned the trigger well wall so that a curved hammer like the Timney would have clearance to reach the firing pin. Huston, we have contact! Timney trigger impacts the pin as you can see below with about .010" of clearance left to spare. When swiveling the lower on the takedown pin with the hammer released, the face of the hammer rubs against the firing pin as you bring them together; indicating good contact between them when the hammer is released. As a side effect, the little wall between the trigger well and the slot for the magazine release button is slightly thinner than the one on the AR-15 (for reference, the same wall in the AR-15 is about .022"). I hope its thick enough to not be fragile and do the job of keeping gasses out of the mag release button.



While experimenting with the BHO, I used a 3/4" long #6-32 screw to install the existing BHO arm and foot to my last intact lower print. I used a corresponding nut to secure the BHO foot to the assembly. I did this to find ways to implement BHO without the need for new parts. The nut between the BHO arm and foot allows me to actuate the bolt release like I do on the normal lower. The nut is .1075" thick and is pressed by the BHO spring due to the location. I might need to ream or widen the hole for the BHO arm, as the bolt release feels very stiff and I can't tell how the spring works in this arrangement.



Here is how the screw and nut look from below. Normal #6-32 screws work on your BHO arm in your standard lower by the way. I may need to widen the slot for the BHO foot a bit more, since the edge of the nut contacts the wall of that slot and might get in the way of tightening the screw.



The screw is a bit too long, I'll try to use a shorter screw and see if that eliminates the gap that you can see from here. I would rather not have to make a new BHO arm (though that would be optimal) but doing things like this should make the system simpler and less dependent on proprietary parts.




So what I need from the community is thoughts on how I got the BHO feature to work, how many slots are appropriate for the rear picatinny rail, and if the lower should be shortened to make it lighter. I can afford to make it another .25" shorter but I'm starting to get concerned about the long-term strength of the lower. Below is a mock up of the AR-XCR lower as 9.5" long.



I feel like I am close to being done. I'm going to dial in on these last details and work on making sure this thing can be machined easily. Upon request, I can send STL files of the current design (in 9.75" or 9.5") for those with 3D printers to evaluate the design on their own.

Just when I thought I was almost done, it pulls me back in!

I printed a new lower based on the latest design and a few old problems came back into the forefront that begged my consideration. The rear beaver tail was just enough for my Magpul grip but the generic grip I got for my Mac Daddy would not fit, so I'm shortening the rear tail a considerable amount to allow for universal grip compatibility.

The trigger well is just wide enough for my Timney trigger. I'm planning on adding about .05" to the length anyway, again to ensure universal compatibility just in case your trigger pack is longer.



The long standing problem with this design has been reconciling the distance the finger of the BHO arm sticks out and the space of the rear lip of the magwell. Right now, the finger of the BHO is just enough to reach the magazine follower, but if you move the magazine just a bit, the finger can miss the follower. The standard lower of the XCR has the finger much further into the follower, as indicated by the black marks on all my magazines. In order to ensure compatibility with magazines, I'm moving the BHO hole .05" forward so that the length of contact between the BHO and the magazine is consistent.



Moving the BHO has the effect of reducing the amount of space available so that the BHO spring and BHO arm can locate near each other to fuction, aswell as thinning out the wall between the BHO hole and the mag well. In order to preserve functional spacing around the BHO, I'm going to have to thin out the rear wall of the mag well to about half of what it is on the standard lower like shown in the photo. The magwell should still work, it just bothers me that the rear magwell wall is much thinner than the one on the standard lower.



Widening the trigger well and moving the BHO hole has had the effect of thinning out the wall between the trigger well and the mag release slot. You can just see on the print how thin the wall is. Fortunately, the mag release arm is long and tolerant of where we locate it horizontally; so I plan to move it about .010" so this indicated wall can be thicker and more of the mag release head can close down on the locking slot on the magazine.



I've already made some changes to the CAD model. The latest model is below with the BHO placed on the new hole location and a magazine to indicate how far the finger reaches into the follower. It looks more like what the standard lower is able to achieve. The BHO head and finger also sit about where they should be on the standard lower.






The below image shows the cost of moving the hole. What's also important is having enough space so a 1/8" BHO spring can slip in there and be actuated by the BHO; too narrow and the spring won't sit in there. You can also see that the rear lip of the magwell is much thinner now; it's about .080" wide when on the standard lower it is more like .130". I don't get how it works like that on the standard lower, but at this point I really need this design to be functional. The sketches of the holes illustrate the spacing needed between the BHO and the associated spring.





I'll have to bump around a few features again and do another print. While the next design fixes these functional issues, I'm hoping the compromises that make the lower thinner in some parts don't affect reliability or the aesthetics of the lower.

Using a nut and longer screw for the standard BHO isn't going to work on this lower. One last simple idea I'm going to try is to print a threaded plastic sleeve that goes between the BHO arm and the screw on the BHO foot that extends the reach of the standard BHO so it can work on the AR-XCR. If that doesn't work, I'm really going to have to build a custom BHO arm that is longer to work on this lower. Any CNC shop should be able to make it but building small parts on CNC can be a risky and expensive job.

In other news, I found a local FFL 07 CNC machinist who have a machine I can hire to make the first lower. I'd like to be close to the action so I'll be contracting them out for the job. I need to think about what kind of logo I want to laser etch on the roll mark section that I can use to finish off this design!

I made an extender for the BHO that allows you to use the original BHO arm on the AR-XCR. It basically extends the length of the BHO by .29" and still function like normal (without her noticing!). The hole inside this extension is just wide enough for the screw to clear through and screw into the BHO arm above it. The fingers at the top of the extension go around the bottom of the BHO arm to help locate the assembly and prevent any rotating as you screw in the bolt.


Here it is installed on a previous design AR-XCR lower with the BHO. The BHO now seems to work comfortably when assembled with the magazine; there is some friction but that could be due to plastic rubbing on plastic.


A bit if a gap seen when the BHO is relaxed and lowered. I don't think this is a problem that needs fixing; the standard lower has a similar gap, though it is slightly larger in the AR-XCR.


The BHO extension seems to work as intended until I can get it tested on an aluminum AR-XCR lower. I'm considering on including a couple of these with every AR-XCR lower order along with longer, #6-32 x 1/2" bolts as a cheap and easy way to integrate the AR-XCR into your system.

I've taken the time to design a longer BHO purpose built for the lower. Hard to tell from these computer-generated models, but the longer BHO arm is .29" longer from the bottom than the regular one.


According to the model, the new BHO arm sticks out in the same position as the standard lower set. I'll have a machinist build one of these to test with the AR-XCR; if any of you are like me, you'd probably want to have a metal BHO arm anyway to go with your AR-compatible lower.


I'll examine the old prints and data mine them some more before I print a new set with the latest updates. I really hope I can be done with designing after this.

So I noticed with my -M that the lower has chamfered (angled) edges around the contours of the lower versus the embossed (rounded) edges on my -L. I presume that the chamfered edges are what are being featured in all new models of the XCR lowers.


My intention is not to flood my inventory with a bunch of different models; I'm only interested in replicating the style of the XCR as closely as possible. Making chamfered edges would be much easier to machine on a CNC and give the rest of the lower a more clean finish because I won't need to use a bullnose endmill or round mill for the edges. It won't take very long to edit the model to display chamfered edges, but I'm taking my time anyway due to it being tax season and I'm doing lots of research to get them done. I just wanted to know if people preferred the embossed edges or the more recent chamfered edges. Once production is underway it's all or nothing; I'm not producing two models with different edge types.

One more thing: my aformentioned local FFL gun shop had a HAAS Mini Mill that they were using to make Glock slides, but didn't have the skills to produce anything else. I'll try to hatch out a deal with the owner so he could charge me time to use his machine to make the AR-XCR. I don't have the cash yet to get my own CNC machine as capable as this, so we have to resort to this. There is also a local anodizing shop I can take my lowers to get mil-spec hard coated too. After I finalize the CAD model, I'll draw up the tool paths so I can make this machine cut out the lower out of the aluminum I provide.