M*CARBO Brotherhood

AR-15 Buttstock Adapter For Your SUB-2000?

Now that’s some creative thinking. I do hear my dad in the back of my head saying use the K.I.S.S. (Keep It Simple Stupid) method though. Why not make basically the same stock but extend the blade down a couple inches and the L.O.P. an inch or two longer with a couple more holes for more adjustability for those of us who are big. That way it would still use the same locking mechanism, allow for a longer length of pull and a better cheek weld.

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Definitely an interesting design, though Chris wants anything they do to still lock folded.

Overall, this is my most desired upgrade, and I think a very logical one once the folding sight mount is shipping. A padded and ergonomic stock replacement, especially one with a mount for a spare mag, would really complete the gun for me.


I’m not endorsing the particular engineering or design that Backpacker used. But the idea of coming off the bolt tube at a slight angle, or even compound angles below the bolt, rather than straight off the back seems to give a lot of buttstock design flexibility, and done right would allow preservation of the existing buffer cap and latch so the firearm still folds and latches as designed. Those criteria are deemed essential for most S2K mods and this approach seems to address and remove those elements as limitations on any buttstock design. It also would change the sight geometry, though by how much and whether it is an improvement would depend on the severity of the angle. Using multiple angles could even allow the stock to stay close to an even plane with the barrel if you wished. See rough sketch below. Note that the OEM stock provides some protection for the sight that may be lost with any stock modification. One solution to that might be to add an extension to the buffer into which the sight recesses when folded. I’m not an engineer or gunsmith and may be missing something obvious but the point is to get the ideas out there so smarter folks can run with them if they make any sense.



I’m gonna need you to build me one of these mounts. Please.

@JoeFridaySays, well done. I’ve been thinking on this since you posted yesterday. You coulda stolen that drawing out of my cerebelum. Well done and I think it’s what the Doc is prescribing.


I’ll likely mock one up using the rimfire, 1", offset scope ring from my junk drawer. I’ve gotta buy an AR buffer tube and buttstock. It will look just like a 2nd grader followed @JoeFridaySays drawing, but I’m bet’n it’s comfortable on the shoulder.


I’ve (belatedly :man_facepalming:) realized that this idea is distinct from separate “replacement buttstock” ideas, and the point isn’t a stock but the ability to mount whatever of the countless AR stocks you prefer. Obviously there’s a lot of value there. Mostly likely this design will add a little length to the overall design, but I don’t know that that can be helped, and a shorter stock would mitigate that.

Honestly I’d prefer a straightforward replacement with better shoulder comfort/less bruising (and ideally a mag storage option, though a freestanding solution there would also work). That’s just my perspective. That said, JoeFriday’s prelim design looks good and would solve some issues, and offer more customizability and options.

We’ll see what Chris and the team come up with. I’m certainly gonna be interested.


Revised Drawing: I realized with a little more thought that any space between the mount and bolt tube is a weakness and not needed. The rear of the mount should partially wraparound and cradle the bolt tube as much as possible for stability and strength. You could even add ears for the stock pin to better anchor it. The drawing below shows a side view and a cross section from the rear of a revised version that would do that (obviously not to scale). Again, I leave it to the experts to figure out if it would work, but it sorta makes sense.



I posted a modified design this morning. I doubt you can mock it up exactly with off the shelf components, but I think it is far more robust than what I slapped together and posted as a concept for discussion yesterday.
Happy tinkering!


I shoved an AR buffer tube and buttstock in my Primary Arms wishlist. I’m going to swing through and grab’m later today so I can bring’m home for cut’n, drill’n, tap’n and attach’n.

I don’t own an AR, so I have never actually examined these components. Looking at your drawings, it’s pretty clear that I will need to slot the top of the AR Buttstock to get it to the factory Kel Tek LOP at its closest setting. I will also likely need to chop the buttstock to fix interference with the s2k charging handle. If I have to give up adjustability and go with a fixed LOP, I will. This mod is appealing because it drops the recoil pad to where my shoulder actually wants it to be when I have a sight picture.

I just read the descriptive text on your drawings. I’m actually going to mount an AR buffer tube to the s2k bolt tube using offset scope mounts. I’ve got a long wingspan, but I’d choke on the LOP created by mounting a buffer tube to the generic buttstock mount. I want the buttpad near where you have located the mount… So, I like your early drawing better. And, I will be chopping as much of the threaded end off the tube as I can manage.

Also, I just rooted through my junk drawer. Turns out the offset, rimfire rings I thought to use are holding glass on my Papoose, so I will also be shopping rings.

I gotta laugh 'cause I just painted the plastic bits kahki/fde/koyote/tan/the-color-that-never-really-matches-up. But, I still have most of a rattle can left so if I like the result - I can make it work.


I look forward to seeing the results. I’m especially interested in your assessment of dropping the stock so you don’t have to lower your head relative to your shoulder as much to get on the sights.

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Well Done @JoeFridaySays! This is it!

I’ll see what we can come up with but the concept is spot on! Excellet work!


Thank you sir. Glad you liked it, but remember I was just tweaking an idea someone else had posted years ago and can’t take credit. See the revised drawing I posted earlier today which is probably a better and more robust approach. I will also send you privately another that may minimize the length problem.


Picked up my buttstock and tube en route to my first job. Still back and forth between mounting a picatinny to the AR tube versus building a rimfire dovetail by tapping the tube for something with an 11 or 12mm bolt head and filing in a groove for it to catch. Got Academy and Bass Pro convenient on my way home, so I’ll let their inventories decide.

Funny to me that the dude working the floor at Primary Arms had never seen a Sub 2000 - all of there employees carry ARs slung on their chests. I made a mental not that should trouble follow me in the door, I hope they can handle it 'cause I’ll be ducking for cover with fingers poked into my ears.


I’d tend to agree with you. A little taller so it’d fit better against the shoulder, maybe 1 to 1.5 inches longer with a couple more holes for more adjustability for length of pull and a smoother shoulder face for more comfort.

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I agree that a straightforward replacement with the features you describe would be a great option, and if I had to choose would prefer that to an AR adapter. But the two do not have to be mutually exclusive and I posted what I did only as a possible solution to the problem posed by the thread - an AR buttstock adapter for the Sub2K. Like you, I’ll be interested in seeing what the team comes up with.


For sure! If Chris and the team want to do both an adapter and a straightforward replacement stock, I’m sure there are plenty of people who would have a preference. And either is gonna be an improvement over the stock…stock. Something like your AR adapter appeals to those who have plenty of stocks/a specific AR stock they really like, and shooters who don’t mind adding a little length (or really want/need the height and extra length).

Others, like us, might just prefer an improved and more ergonomic replacement. I don’t mind the cheek weld terribly (and it works for my 45degree offset crosseye dominancy solution–which lets me use the red dot with my dominant eye and irons with my other from the same cheek weld). But I’d still drop a pretty penny for something more comfortable and with improved utility.

Either one is gonna continue turning the Sub into a very different beast than the factory model, which is cool with me. It’s a neat and interesting little gun, and the weirder it gets the better haha


Dred and The Three Laws made some smart observations about the two earlier drawings I posted that got me thinking. So I prepared the additional drawing below that tries to incorporate the best features of the first two and address some of the points they made. I don’t know if it is practical to separate the upper and lower sections with an adjustable slide, but I also threw that in for consideration as there seems to be some interest in greater adjustability of the stock for a longer LOP than what is offered by the OEM version. An alternative to an adjustable slide would be to simply keep the bottom part short and then have extension adapters for the adapter for those wanting a longer LOP. That isn’t as wonky as it sounds since the extension adapter would basically be a secure tube within a tube.

Of course, any stock modification will raise the issue of where the shooter’s cheek ultimately will rest and how the adapter should account for that. I don’t know how you deal with that other than mocking it up and testing.


Why not make a new buttstock? An altered version of the strike industries viper or the minamalist buttstock maybe? Something that had rubber plating but small sleek and you still feel like it doesn’t turn your gun bulky.


First off, it’s possible I missed something I’ll admit I haven’t read the entire thread, but as far as I have seen I haven’t seen anything that works good that doesn’t involve modifying the tube somehow. With that piece being considered the firearm I think a lot of people want to leave it alone. I know the idea of this adaptor is so people have the ability to install the AR buttock of their choosing but it just doesn’t seem to be any reasonable ways to do that. I am onboard with an aftermarket stock for my S2K. Here’s an idea: as everyone knows the original stocks on the S2K’s come with only 3 positions which involves which involves removing a pin to adjust, a 6 position telescoping stock like that on an AR adjusted in the same way (a lever) would be nice. I like the idea of it being based off of MFT’s minimalist buttstock and also the ability of storing more mags on it. Redesign a piece that goes over the tube and attaches using the hole where the pin is that all allows a redesigned stock to be adjusted with a lever instead of removing a pin. The redesigned stock also allows the original locking mechanism to work. So pretty much the same except telescoping and a rubber buttpad.


So any new on who’s going to design this butstock seen alot of ideas someone make it

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Ok … just surfacing to report progress. I have none to report. I’ve altered my target 'cause I decided I couldn’t live with the wonky layout generated by the scope rings.

But, Sunday, some guy posting as casper3oaks over at the KTOG posted this:

Casper3oaks at KTOG

And I’m in. IMO the solution is elegant enough to copy. My test tube is currently honed out for an interference slip over and shortened. This morning I marked the tube where the front sight will stow. Once the access/stowage is cut, I will begin drilling the tube for the bolt retainer pin. The buffer tube will actually be fixed in place on the Sub 2k bolt tube with this pin ala factory buttstock, so … once the tube is drilled, the next step will be grinding the pin down near flush with the AR buffer tube.

Casper3oaks rounded the little tube side latch point and glued it to the AR buttstock to hold it in the folded position. I’m not sure I will follow the plan that far. I’m thinking that the ability to tuck the front sight for a nearly complete fold may serve my needs. If not, fixing the latch to the AR Minimalist stock will allow me to lock the stock using the 6 position adjustment.

Here is my dilemma. I anticipate ditching my irons when I install my M*Carbo Flip to Side Optic Mount. If I ditch the front site, I won’t need to stow it and I’ll be wanting a cutout free cheek weld platform. My plan is get the buffer tube cut and prepped but try to hold out finishing the cuts and glue on the Minimalist until my final sighting solution is settled.

I will cut the front off of the Minimalist stock to prevent interference with the charging handle. Hopefully I can retain the QD Sling Mount socket.

And Edit to Add: I’m working with a Mil-spec tube and stock. But, hindsight leads me to confirm that the honing process would have gone much faster had I chosen a Commercial spec tube and stock. The larger bore dimension on the Commercial would simply require less material removal.