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Discussion Starter · #41 ·
They are here (but hosted on YT IIRC)....but the search engine sux here. When I get time, I'll try and look for it.
Thank you for that. I've been looking around myself but it's possible the video was posted in a thread where it wasn't the main topic. I see Mountainman had some XCR videos but it doesn't appear to be him who made the mod.
 

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Here it is:

 

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Discussion Starter · #43 ·
Here it is:
Fascinating, this was a relatively low-tech method to do what I intend. Much like how I imagined, abran had to delete the bolt catch to make room for the AK mag catch. You can see in the videos he posted that the last round BHO no longer works and that he's using the side charging handle to rack the next round, things I was hoping to avoid.

I only explored this branch of the lower development thinking it might be easier to do and in higher demand than doing the AR FCG. Now that I know it's been tried, I still think that is true, but I think I'll shelve this idea for a while after I reconcile what I want to do with the BHO that we're all fond of. Come to think of it, there might not be enough room on the side for an AR bolt release to be there (the side charger in the way). Using the Bren 2 mag is a very compelling idea, and after a little research on the issue, I doubt CZ will discontinue making the mags because their CS has been very responsive (they'll either fix the problem or pretend it's fixed and keep it moving) and 7.62x39 is a popular round for CZ's customers in that part of the world.
 

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Fascinating, this was a relatively low-tech method to do what I intend. Much like how I imagined, abran had to delete the bolt catch to make room for the AK mag catch. You can see in the videos he posted that the last round BHO no longer works and that he's using the side charging handle to rack the next round, things I was hoping to avoid.

I only explored this branch of the lower development thinking it might be easier to do and in higher demand than doing the AR FCG. Now that I know it's been tried, I still think that is true, but I think I'll shelve this idea for a while after I reconcile what I want to do with the BHO that we're all fond of. Come to think of it, there might not be enough room on the side for an AR bolt release to be there (the side charger in the way). Using the Bren 2 mag is a very compelling idea, and after a little research on the issue, I doubt CZ will discontinue making the mags because their CS has been very responsive (they'll either fix the problem or pretend it's fixed and keep it moving) and 7.62x39 is a popular round for CZ's customers in that part of the world.
Yeah, but frankly LRBHO isn't that big of a deal if you're at all used to AKs. It's even less of a deal if the lower can take Yugo mags with the BHO feature.

But I agree. This should be tabled. It's not the primary focus of your lower.
 

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Discussion Starter · #45 ·
Yeah, but frankly LRBHO isn't that big of a deal if you're at all used to AKs. It's even less of a deal if the lower can take Yugo mags with the BHO feature.
I didn't know that about the AK actually, but in that case I guess it would be acceptable to have a lower without the LRBHO. I'll keep that in mind.

I'm treading new ground with this project so I'm starting off by keeping my design goals very narrow. This time I'm going to just make a copy of the XCR-L lower but with a built-in picatinny rail. I'm sticking to the basics so I can really understand the style of the XCR and limit the number of things that can go wrong by sticking as close to the things that I know work as possible. Once I can get this 3D printed and I've confirmed that my process works, it should be a cinch to edit this base model into other designs for the XCR.

Here's what I've managed after a couple hours of work. I've made a 2D scan of my lower and super imposed that image over the sketch I've made based on measurements I've made with my calipers. I really need to get a radius gauge and other precision measuring tools to get these contours and fillets. I might use a pen and trance the lower and take measurements off of that. As I said before, I'm making all my data publicly available as I make them so others can make one themselves once it's all finished.
 

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Discussion Starter · #46 ·
Latest update on the lower, it looks a lot more like the lower we have. I still have to get more dimensions, a lot are still undefined, but I'm reaching the end of what my digital caliper can do without more measuring tools. Of note is that the trigger well of the default XCR lower is .925" deep, while that on an AR is .950" according to specs. Nate's information that the XCR lower being .25" shorter than an AR holds true in this case, but there is a big difference between .25" and the .29" I found while measuring the AR triggers in the Trigger thread. I'll need to look at these dimensions very carefully to see which works. To compare, I've super imposed a model of the M4 lower on top of my lower based on the location of the grip - I'm a bit surprised how things are matching so far. Anyway, I measured the height difference between the two and I see about a .31" difference.

The pic rail is pretty long, it has 6 slots along it. I also rounded out the corners of the pic rail to match the rounds on the lower's rear. I wanted to know if that looked good, and would that mount OK with pic rail-mounted stocks?

I noticed that the design of the XCR is patented and looked it up. It looks like the patent is on the rifle's design itself and operation (the patent: US20050262752A1 - Firearm - Google Patents). I was wondering if this would be a problem if I released plans/CAD models of the modified lower online like this? Maybe Mr. Robinson wouldn't want me releasing a reverse-engineered version of the original lower, but the ones designed to take other FCGs and magazines are all my work.
 

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Latest update on the lower, it looks a lot more like the lower we have. I still have to get more dimensions, a lot are still undefined, but I'm reaching the end of what my digital caliper can do without more measuring tools. Of note is that the trigger well of the default XCR lower is .925" deep, while that on an AR is .950" according to specs. Nate's information that the XCR lower being .25" shorter than an AR holds true in this case, but there is a big difference between .25" and the .29" I found while measuring the AR triggers in the Trigger thread. I'll need to look at these dimensions very carefully to see which works. To compare, I've super imposed a model of the M4 lower on top of my lower based on the location of the grip - I'm a bit surprised how things are matching so far. Anyway, I measured the height difference between the two and I see about a .31" difference.

The pic rail is pretty long, it has 6 slots along it. I also rounded out the corners of the pic rail to match the rounds on the lower's rear. I wanted to know if that looked good, and would that mount OK with pic rail-mounted stocks?

I noticed that the design of the XCR is patented and looked it up. It looks like the patent is on the rifle's design itself and operation (the patent: US20050262752A1 - Firearm - Google Patents). I was wondering if this would be a problem if I released plans/CAD models of the modified lower online like this? Maybe Mr. Robinson wouldn't want me releasing a reverse-engineered version of the original lower, but the ones designed to take other FCGs and magazines are all my work.
Alex probably won't appreciate it or like it, but the fact is patents only require a 10% change to circumvent in most cases. In this one, the FCG is a major difference. I don't think he can legally stop you frankly.
 

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Discussion Starter · #48 ·
I don't think he can legally stop you frankly.
If this makes it big, I'm half-expecting a cease and desist order anyway, lol.

Quick update on the lower, looks a lot tighter now that I'm able to measure angles and radiuses on it. The magwell and grip mount appears to be spot on, hope everything else is too.
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Another overlay of the M4 lower on mine, lined up with the pistol grip mount. The beaver tail on the XCR is recessed back more than the M4 lower, but that correlates with what I've seen in reality. Overall, it looks very closely lined up. I'm concerned that the pin for the trigger on the XCR appears to be more forward than the AR's. The XCR trigger pin is located on the center of that lobe that's above the mag release, but it looks like the AR's is just behind that lobe. I was hoping to just extend the height of the XCR lower and not worry so much about pin locations, but it looks like I might have to work with this.
Font Rectangle Gun barrel Auto part Trigger

Next step is to disassemble my lower and get those final dimensions. Once the pin holes are in, I can start printing out prototypes for fitness testing.

Kinda regretting not getting some aluminum before prices climbed up this month. Considering prices and how big plate stock are, I'm really considering on building two lowers at once from one plate stock; the lowers facing each other in a yin-yang, 69 position as I mill them out. It'll save a ton on scrap and let me have two cracks at the project, just in case.

The CNC machine I'm getting to mill this is the Shariff DMC 2; it's being held up due to shipping lead times but should be getting around to me within a month or two, so I'm not in a huge hurry to have the lower completely designed until then. And other than hole locations and fine-tuning of the shape and fitment, it's not much work left now.
 

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If this makes it big, I'm half-expecting a cease and desist order anyway, lol.

Quick update on the lower, looks a lot tighter now that I'm able to measure angles and radiuses on it. The magwell and grip mount appears to be spot on, hope everything else is too.
View attachment 15631
View attachment 15632

Another overlay of the M4 lower on mine, lined up with the pistol grip mount. The beaver tail on the XCR is recessed back more than the M4 lower, but that correlates with what I've seen in reality. Overall, it looks very closely lined up. I'm concerned that the pin for the trigger on the XCR appears to be more forward than the AR's. The XCR trigger pin is located on the center of that lobe that's above the mag release, but it looks like the AR's is just behind that lobe. I was hoping to just extend the height of the XCR lower and not worry so much about pin locations, but it looks like I might have to work with this.
View attachment 15630
Next step is to disassemble my lower and get those final dimensions. Once the pin holes are in, I can start printing out prototypes for fitness testing.

Kinda regretting not getting some aluminum before prices climbed up this month. Considering prices and how big plate stock are, I'm really considering on building two lowers at once from one plate stock; the lowers facing each other in a yin-yang, 69 position as I mill them out. It'll save a ton on scrap and let me have two cracks at the project, just in case.

The CNC machine I'm getting to mill this is the Shariff DMC 2; it's being held up due to shipping lead times but should be getting around to me within a month or two, so I'm not in a huge hurry to have the lower completely designed until then. And other than hole locations and fine-tuning of the shape and fitment, it's not much work left now.
If you make one I'll order one but keep us updated please, the world shall known you lol
 

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Discussion Starter · #53 · (Edited)
Awesome work
You're awesome work!

If you make one I'll order one but keep us updated please, the world shall known you lol
I'm just focused on designing this one lower and there are significant challenges that need to be resolved just to build one for myself. I can probably have the design finished and tested in a couple months, but I'm rethinking the kind if CNC machine that I'm going to need to pull this off (and those things ain't cheap). I'd like to make all sorts of announcements but my track record for completing personal projects has been very slow (but improving a lot in the last year) and I don't want to over promise. I do plan to make this design available to others in some way once I've proven that it works. However, applying for an FFL in order to sell "firearms" like this is no small feat and not something you'd do as a hobby over the weekend.

If the bren2 7.62 isn't stillborn, if I get you dimensions for the 2nd gen 7.62 mags think you could work something up to see if we could make it fit?
I'm sure we can, but I think we'll hold onto that idea and open a new thread for that project after this one is complete. Building and testing off of the x39 platform will be a significant investment for me not just for XCR parts, but I might end up having to buy a Bren 2 just to get the magwell design and to have a control group for the subsequent experiments.

Actually, what I was planning to do after this project was construct a bullpup lower receiver for the XCR-L upper. There was gathering momentum online for this idea and I have some ideas to make it work, but I'm not investing significant time to it until after I learn how the standard lower works for the XCR.
 

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It's basically stanag, but slightly larger by 0.1" on the sides and front. The curve also starts higher in the body. Feed lips and magazine release position are in the same spot. I can get you exact dimensions. I was thinking of prototyping a printed lower just to see how it would feed with the xcr bolt
 

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Discussion Starter · #55 ·
First draft of the model of the default XCR-L lower. Looks damn close, but there are a few features and measurements that I can't quite reconcile right now.
Art Auto part Automotive exterior Font Automotive window part


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Got the magwell flare, the BHO section and the grip screw hole, more or less where it needs to be. Made the clearance for the bolt carrier over the rear; looks shorter than in real life but it really only goes out 1" anyway.
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The sides:
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I'm concerned that a lot of the minor dimensions in the main body remain unconstrained/unknown. Some features may not be where they look like they should be right now.
Air gun Trigger Gun barrel Gun accessory Auto part

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The above super impose with an AR lower shows that the FCG holes are pretty close already. I'm concerned that the hammer pin hole is too far forward than what is actually on the XCR lower. However, the location of that hole was determined from my measurements and should be accurate to real life.

I think I've exhausted what I can do with modeling what I find with my measurement tools. Time to start printing out examples and checking how it all fits on the actual rifle. After a round of corrections, it'll be simple to raise the height of the lower to match what is required for the AR FCG to make that lower. My calculations show that my lower is 0.2966" shorter than an AR's lower.

Any pointers about the design so far is welcome.
 

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Discussion Starter · #56 · (Edited)
I'm starting to notice that with other stock 1913 adapters, the rails are tapered since most stocks/braces don't use more than 3-4 slots and to provide some height for the cheekweld. I decided to do some cuts on the 1913 rail, how does it look?

Font Automotive exterior Auto part Rectangle Circle


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Again, with rounds:
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Automotive exterior Font Auto part Map Event

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I'm starting to notice that with other stock 1913 adapters, the rails are tapered since most stocks/braces don't use more than 3-4 slots and to provide some height for the cheekweld. I decided to do some cuts on the 1913 rail, how does it look?
Looks better, BUT......

The XCR has a very real difference in the cheekweld height vs. an AR. You may need that upper slot to properly accommodate more shooters. In my case, I run really low optic mounts (opposite of the jawline high optic trend this week) with a very repeatable cheekweld. I doubt anyone who shoots long range much will argue that repeatable cheekweld isn't critical to accuracy.

Also, what's the minimum interface length for like an ACR stock (which is very popular on this rifle)? The aesthetic aspect of the stock looking like it belongs with the rifle is important to some.... Having a shorter interface/pic rail with a stock that hangs off of that interface could be visually unappealing.

Your call, just my two cents.
 
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Discussion Starter · #58 ·
The XCR has a very real difference in the cheekweld height vs. an AR. You may need that upper slot to properly accommodate more shooters. In my case, I run really low optic mounts (opposite of the jawline high optic trend this week) with a very repeatable cheekweld. I doubt anyone who shoots long range much will argue that repeatable cheekweld isn't critical to accuracy.
I see. It's easy to restore it to how it was before. I'll probably print both receivers and see which ends up better.

Before anyone says I forgot to put the slot for the retaining pin for the receiver bolt, I didn't. I just didn't know if I could make the cut with a band saw or an end mill. I think I'll try for a thin end mill.
Gun accessory Rectangle Gun barrel Trigger Composite material

What I did almost forget was the hole for the safety selector detent.
Rectangle Machine Auto part Automotive exterior Gun accessory

From the bottom it's in the right location, but from the side, you can see that the hole is supposed to open at the bottom quadrant of the safety selector hole but is shifted instead. I can't get it close to that spot without changing around a bunch of measurements I made. It suggests I have the safety selector in the wrong location, but if you take a look at the back, the safety hole is where it roughly should be relative to the receiver. I have issues like this in other parts as well. I guess I have to keep sharpening these dimensions.

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Beauty shot of the lower's right side:
Outdoor shoe Downhill ski binding Handgun holster Auto part Font
 
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