ACR and 1913 rear stock adapters for XCR lowers

1435 Views 19 Replies 7 Participants Last post by navalbeaver, 4 d ago

Chowwow

Discussion Starter · 6 mo ago (Edited)

While waiting on my 3d Printer to print some things, I took some time to design my own stock adapters for a 1913 rail and for the ACR folding stock. People who have been here for a while will know that a few people have made these adapters over the years, with the most recent designs from Lucky Irishman who appears to be withdrawing from the business. To fill that void, I've decided to make my own. While the ideas are similar, I didn't use any of Lucky Irishman's data to make these; just measurements and observations on what I've learned about these platforms. I would like to give credit to 3D print group AWCY? for releasing the CAD models of the ACR folding stock end to the public; this project was made much more easier thanks to their hard work.

My idea is to produce two options for each adapter. One, a full-featured, CNC-machined aluminum adapter that has QD sling points and threaded holes both to screw into the XCR lower and to screw in the sling loop in the original ACR folding stock. Machining the 1913 rail is very simple, however, I can't think of an easier way to machine the ACR folding adapter without having to invest in a 5-axis machine. The other, more tangible option now, is to produce simplified and affordable 3D printed adapters for people who need one right away and don't need special features. While they won't be metal, and you have to be weary of keeping them in a hot car approaching 140 degrees F, they will be made of the best material available and they are all very sturdy and simple to install.

CAD models for the ACR folding stock adapter:

Font Auto part Cylinder Automotive exterior Line art

CAD models for the 1913 adapter:

Rectangle Automotive tire Auto part Automotive exterior Font

The ACR adapter fresh off the printer:

Automotive tire Automotive exterior Wood Hood Asphalt

The 1913 rail adapter:

Automotive exterior Bumper Wood Glass Flooring

Audio equipment Electronic instrument Carbon Camera accessory Auto part

Support material removed:

Automotive lighting Automotive tire Automotive exterior Tints and shades Audio equipment

The nice thing about the 1913 adapter is that it replaces the need for a pistol endcap or fixed stock adapter.

Tints and shades Composite material Fashion accessory Rectangle Auto part

Both adapters have clearance holes so a 1" long, 1/4-20 thread screw can reach through and screw into a nut on the other end.

Font Auto part Fashion accessory Rectangle Carbon

I still need to install these things onto my rifle and test them for strength. Will report back when I get the chance to do so. Eventually, I'd like to make stock adapters for a buffer tube extension (so you can use AR stocks on the XCR) and an adapter for the Magpul Zhukov stock (I'll need a Zhukov first). Any ideas or critiques are welcome.

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roflcopters

· #2 · 6 mo ago

I have a run of the first acr adapters with the bottom qd. You'll want to make sure the charging handle can clear and the folding angle/depth is right so you can manipulate the controls with it folded. I'll be throwing my stock on for paint once my upper arrives so I can give you measurements.

Really you should have a qd on all of the adapters

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navalbeaver

· #3 · 6 mo ago (Edited)

I'd shorten the 1913 as much a you can. LOP can be a little long with the current options and yours looks longer

Eta. Good stuff.

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joshobean

· #4 · 5 mo ago

Great work sir! Looks incredible and hopefully one day 3d printers and materials will match the strengths necessary for hard core use.

I tried 3D printing when COVID hit in 2020. Sadly I went with a cheap Chinese printer and probably was too ambitious for it's capabilities. I tried several filaments, from PLA to PETG and finally nylon.

What printer and materials did you use for this project?

Chowwow

Discussion Starter · #5 · 5 mo ago

joshobean said:
Great work sir! Looks incredible and hopefully one day 3d printers and materials will match the strengths necessary for hard core use.

I tried 3D printing when COVID hit in 2020. Sadly I went with a cheap Chinese printer and probably was too ambitious for it's capabilities. I tried several filaments, from PLA to PETG and finally nylon.

What printer and materials did you use for this project?

If you look around the 3DP2A space, there is a lot you can do with the current tech, and new designs are pushing the bounds of what can already be done. A lot of the ideas I had thought of earlier in the year have already been met or made obsolete already, so it moves very quickly. I'd like to do more in this space but if I were paid to spend time making designs for various guns like I do for my regular job, I'd be a lot more productive.

The monster I'm using right now is the Creality CR-10 S5, currently the largest printer that you can buy in the market for $700, as far as I know. Following that, I bought nearly $700 in additional parts and an enclosure to get it to work consistently as it does now. I got it so I would not be limited on space for what I can print in the future; most popular printers like the Creality Ender 3 are not quite large enough to print the lower for the XCR, but the CR-10 S5 has been able to put them out on a regular basis. I was printing with a generic PLA for model prints, but for these latest prints I've moved on to Inland's tough PLA, where I hope to produce models that are not only accurate to CAD but strong enough to perform fitness testing and even field deployment. I've been printing on a bed heated to 60 degrees C, printing on a hot end at 230 degrees C, at 100% infill with 10% fill on the supports, and I've started to print with a brim on the supports to help the models adhear to the print bed and stop warping. The real trick to 3D printing is to fiddle around with your settings to get the results you want with the materials you are using. You'll have to do your research and use manufacturer recommendations to narrow it down.

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joshobean

· #6 · 5 mo ago

Awesome. I had an Ender3 v2. Went through 3 motherboards on it and 2 hot ends, wanted to pull my hair out but I probably started too aggressively and expected too much.

I think I had a lemon. Definitely needed an enclosed space to prevent nylon warping. Even bases and different techniques didn't really solve it.

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Chowwow

Discussion Starter · #7 · 4 mo ago

joshobean said:
Awesome. I had an Ender3 v2. Went through 3 motherboards on it and 2 hot ends, wanted to pull my hair out but I probably started too aggressively and expected too much.

I think I had a lemon. Definitely needed an enclosed space to prevent nylon warping. Even bases and different techniques didn't really solve it.

It sounds like you pushed yourself to far, too fast. I'm still getting acclimated with the materials and tools that I have; it's not perfect but things are working as intended. I worked on using one brand of filament and making my prints as consistent as possible by buying gantry levelers, and enclosure, vibration absorption feet, upgraded T-nuts and an upgraded bed heater. I highly suggest you get back onto the hobby and start small with PLA+ and work on each little thing at a time, before it's made illegal, lol. Honestly, people who think criminals are using 3d printers to get around firearms laws don't know what they are talking about; there is no criminal out there with the patience and money to get into this space who also lacks the self-control to want to enact violence with these things. It's much easier to buy or steal regular guns than it is to leave a large receipt trail of printer and metal gun parts and assembling them.

Onto the design of these adapters themselves, it's not too hard to edit these designs for prime time, but there is curiously little interest in these adapters so far both on here and reddit. Getting these in metal will be the most important goal for this project. I also recently purchased a Magpul Zhukov stock for cheap to make an adapter for that when I'm available, though I expect enthusiasm for that will just be as tepid. Until then, I'll keep working on these but they are not as urgent as the XCR lowers.

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joshobean

· #8 · 4 mo ago

Appreciate the encouragement. It would be good to get back into it at some point. Some of the latest videos such as from Hoffman Tactical on YouTube are jaw dropping and what they are doing with 3D printed AR creations.

I confess my primary interest is in that space, and when I saw the time and effort 3D printing takes, and then what the material my setup supported, I decided it wasn't worth the effort at the time. I would need to invest in better equipment and possibly wait for the industry to catch up to consumer printing of items that stand up to abuse like modern metals and polymers in firearms today.

Having 3 young kids at the time also required me to limit my hobbies. Keep doing what you are doing however. Hopefully one day all this is still legal and we can print practically indestructible, real world tough parts.

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Chowwow

Discussion Starter · #9 · 4 mo ago

joshobean said:
I confess my primary interest is in that space, and when I saw the time and effort 3D printing takes, and then what the material my setup supported, I decided it wasn't worth the effort at the time. I would need to invest in better equipment and possibly wait for the industry to catch up to consumer printing of items that stand up to abuse like modern metals and polymers in firearms today.

Having 3 young kids at the time also required me to limit my hobbies. Keep doing what you are doing however. Hopefully one day all this is still legal and we can print practically indestructible, real world tough parts.

With kids, I know how that feels! Once you get the system dialed in though, these machines just take care of themselves, just how I like it!

I argue the critical point with the tech is now; 3D metal sintering is going to take some time to be cheap and it will be messy, only for garage machinists in the future. Tinkering with your printer IS the hobby when you are messing with these cheap printers; once you get the file you want to print, it's just a matter of pressing "start" and waiting. If you want to skip all that crap and just get what you want printed, I suggest more expensive printers from Makerbot that are about ready out the box to turn plastic spools into art or tools; Makerbot's machines are what businesses and schools use to make reliable 3D prints with no messing around. They start at $1200 - $2000, but if you want to make your Creality or Prusa printer reliable and robust, you need to spend the kind of money on equipment that would get you a Makerbot printer anyway. If you got a lot of projects or ideas, that investment on the printer will pay for itself anyway.

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joshobean

· #10 · 4 mo ago

Sounds like I need to get a Makerbot if I get back into the game, appreciate the recommendation. Back when I was younger (teenager/college) I used to build and tweak endlessly computer gaming machines. Now I have little patience for that sort of thing - and I'm willing to pay to have it now. Probably not a good trait, but hey - is what it is!

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Chowwow

Discussion Starter · #11 · 4 mo ago

I've redesigned the adapters to be a short as possible. The picatinny adapter is less than one inch long now, while the ACR adapter is a little over one inch.

Automotive tire Wood Window Material property Bumper

Wood Window Auto part Font Rectangle

Here's how the picatinny adapter looks attached to the lower. The adapters require a 1-1/4" long, 1/4"x20 screws and corresponding nut to secure it to the lower.

Trigger Air gun Gun barrel Gun accessory Metal

Bumper Gun accessory Auto part Camera accessory Cable

The length of the lower+picatinny adapter versus the AR-XCR lower. It's just a bit longer than the AR-XCR.

Bicycle part Bumper Automotive exterior Rim Auto part

How the ACR adapter looks on the lower.

Air gun Trigger Gun barrel Gun accessory Wood

Bumper Wood Gadget Automotive tire Auto part

How the adapter looks like compared to the AR-XCR.

Wood Gas Auto part Composite material Automotive exterior

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Flskunkape

· #12 · 4 mo ago

Those look much cleaner than the lucky Irishman ones!

joshobean

· #13 · 3 mo ago

Looks very sharp, @Chowwow!

Iandv

· #14 · 3 mo ago

Flskunkape said:
Those look much cleaner than the lucky Irishman ones!

I just got one and it looks much better than all the previous lucky Irish adapters I've seen here. The anodizing is black instead of that purple.

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Chowwow

Discussion Starter · #15 · 3 mo ago

Flskunkape said:
Those look much cleaner than the lucky Irishman ones!

joshobean said:
Looks very sharp, @Chowwow!

Eh, there are typical imperfections that come with 3D printing on them; stringing, pitting, etc. If I do sell these out, I may need to sand and file them down a considerable amount. The thing is, with parts as relatively soft as these, they will tend to get marred easily anyway and they won't look "perfect" in little time. They do seem strong enough to serve their purpose and priced at a point where no one would cry about breaking one or two of these anyway.

Iandv said:
I just got one and it looks much better than all the previous lucky Irish adapters I've seen here. The anodizing is black instead of that purple.

It wasn't just that, but the machining step-overs that gave them a stepped appearance. I suppose that wasn't a big deal and the parts were still functional, though most machinists I've worked with would go the extra little effort to make them appear as smooth as reasonably possible.

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Chowwow

Discussion Starter · #16 · 7 d ago

Printed another set of the 1913 adapters and actually installed one onto my -L. I printed two, one upright (intended to resist pulling forces) and one laying down (to resist shear).

I installed the upright print onto my -L and used a Magpul PRS Lite with a Midwest Industries picatinny buffer tube. I like how the design give you the option to adjust the height of your stock, but where I have it on the photo looks to be the typical place you'd place the stock anyway. I might do another design that rounds off that top and evens out the look.

Close look at the assembly post range trip. When I was operating with the adapter, everything felt fine and sturdy. I then tested the adapter by striking the stock at a wooden wall, like I was going to knock out someone's teeth. Not one impact later and the adapter split along the path of the screw; the stock was still attached and I could still shoulder the rifle, but I knew the adapter was broken.

How the adapter looks now. Pretty good for a 27 grams of filament. When I do another round however, I'd like to try with ABS plastic; that stuff is more ductile and heat resistant while still as strong. If that works well, I would feel more confident at selling a few of these on the side.

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navalbeaver

· #17 · 7 d ago

Looks like you got the fore/aft length about a short as you can go. When I worked with lucky Irish on their adapter I was all excited about the adjustable height but the reality seemed to be the bottom couple of slots were kind of useless. Not sure what else you could do with that space but I find it hard to believe that anyone is going to use a stock that low. Just an observation.

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Chowwow

Discussion Starter · #18 · 4 d ago

The 1913 adapter is based on the rail on the AR-XCR. I didn't think about it much but now that I look at it, I might want to cut down the top most slot to form a slope to make the overall appearance of the rifle look slicker. The Sig Virtus MCX has kind of the same issue and the overall look of the rifle suffers in the same way.

It seems that since Kinetic Development Group discontinued the ACR stock production, there is less demand for that ACR stock and the latest accessories. Combined with the fact that the remaining batch of Lucky Irishman ACR stock adapters are still enough to provide for the remaining stock of ACR stocks, and the fact that the ACR stock is available to be 3D printed. I think I'll be scaling back my operations on developing the ACR stock. I will be keeping the data that I do have but I won't be in a hurry to have them made anytime soon.

As for the 1913 adapters, I'm thinking of making an all-steel version of the adapter with a QD sling point. Those sling points need to be made out of steel for the rigidity; I redesigned the AR-XCR to thread in a steel QD cup due to this. As an adapter however, instead of making an aluminum adapter with a steel QD cup, the whole adapter can be made out of steel for simplicity of manufacture and rigidity. It would be a few ounces heavier but, depending on the steel I use, can be cheaper to buy than an aluminum one. Thoughts?

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Sean K.

· #19 · 4 d ago

Chowwow said:
The 1913 adapter is based on the rail on the AR-XCR. I didn't think about it much but now that I look at it, I might want to cut down the top most slot to form a slope to make the overall appearance of the rifle look slicker. The Sig Virtus MCX has kind of the same issue and the overall look of the rifle suffers in the same way.

It seems that since Kinetic Development Group discontinued the ACR stock production, there is less demand for that ACR stock and the latest accessories. Combined with the fact that the remaining batch of Lucky Irishman ACR stock adapters are still enough to provide for the remaining stock of ACR stocks, and the fact that the ACR stock is available to be 3D printed. I think I'll be scaling back my operations on developing the ACR stock. I will be keeping the data that I do have but I won't be in a hurry to have them made anytime soon.

As for the 1913 adapters, I'm thinking of making an all-steel version of the adapter with a QD sling point. Those sling points need to be made out of steel for the rigidity; I redesigned the AR-XCR to thread in a steel QD cup due to this. As an adapter however, instead of making an aluminum adapter with a steel QD cup, the whole adapter can be made out of steel for simplicity of manufacture and rigidity. It would be a few ounces heavier but, depending on the steel I use, can be cheaper to buy than an aluminum one. Thoughts?

I doubt it's less demand for the ACR stock....it's limited availability and rising cost for any that are up for sale. People aren't quite yet sold on 3D printed for accessories and I personally don't think the technology is quite there yet in terms of it being as strong as a conventionally made polymer part.

As for all steel vs. aluminum with steel insert on the adapter.....I think it's going to be significantly heavier feeling in all steel. It may only be a few ounces in reality, but if one were holding both in hand, the difference would be dramatic. Now will that added weight translate to something noticeable on a full up rifle? Probably not....though the old adage of ounces equal pounds and pounds equal pain would seem to apply.

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navalbeaver

· #20 · 4 d ago

Fwiw,

www.xcrforum.com

Group Buy for MCX stock adapters for the MCX

We need 20 of these to get these going. Group buy price is $100 shipped and anodized. These are going to be made by Lee Majors. I'll be the POC for these so let me know if you have questions. Timeline will be dependent on interest. More pics on page 3 Just for reference. Current "adapter"...

www.xcrforum.com

www.xcrforum.com

All my adapters now are the JMwalker versions. My only complaint is it's a little long.

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