Nick Leghorn for TTAG
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SIG MCX stock adapter
Nick Leghorn for TTAG

One of the biggest differences between the AR-15 platform and the SIG SAUER MCX is the change from a buffer tube sticking out of the back of the lower receiver to having all of that stuff contained entirely within the upper receiver.

It’s a better design, and allows the MCX to have a Picatinny rail on the back of the lower receiver where you can attach your stock instead of needing to screw in the buffer tube. But what about for those who want to put that same skeleton stock on their AR-15 lower? Well SIG SAUER thought of that with their adapter, and having seen a few others using it recently, I thought I would give it a try.

First, a question: why would anyone need this thing? Let me answer that with a story.

I’ve been playing far too much Call of Duty: Modern Warfare lately. Like, an unhealthy amount. And through that game I’ve rekindled a love for my SIG SAUER MCX that lay dormant for far too long.

The thing just looks so dead-sexy short, suppressed, and with its skeleton stock. There was just one problem: I had a pistol version with a collapsible brace, and I had never taken the time to Form 1 the lower receiver.

What I did have, however, was a Form 1’ed SBR AR-15 lower receiver that wasn’t currently being used in any other build. When I was doing a lot more writing here, I SBR’ed just about everything I could get my hands on, including some lowers for projects I still hadn’t figured out. This was one of those lowers.

So with spare lower in hand, my options were to either Form 1 the MCX lower and wait for that whole process to clear, or I could find a way to adapt the existing Form 1’ed AR lower to accept the skeleteon stock.

My solution?

So, while I wait for the paperwork to clear, I grabbed the adapter from the SIG SAUER store and set to work.

SIG MCX stock adapter
Nick Leghorn for TTAG

The adapter is a three piece contraption that bolts onto the back of your existing lower receiver. There’s a threaded plug that fits into the location where your buffer tube would normally go, and then there’s an end plate that is attached to the plug using an oversized screw.

It’s super-simple to install, which makes up for the lack of instructions included in the packaging. It comes pre-assembled, and through disassembling it you figure out how it works and it’s mostly self-evident from there.

Also appreciated is that the same torx wrench needed to install the MCX stock is used to install the plug, so you don’t need to buy new tools for it (theoretically).

SIG MCX stock adapter
Nick Leghorn for TTAG

Once installed the adapter plate does a couple things. As you’d expect, it provides a Picatinny rail section on the rear of the lower receiver where you can now install your stock with ease. At the same time it also keeps the rear detent spring captured for the rear takedown pin, something that the end plate would normally do in an AR-15.

It also adds a handy QD cup where you can attach a sling. It’s only one, unlike the proper MCX receiver that has a pair of QD cups on either side, but it’s a lot better than a sharp stick in the eye.

Beyond being functional, it also improves the aesthetics of the gun. If you just slap an MCX upper on an AR-15 lower you’re going to get something strange looking. The MCX upper is taller than an AR’s to allow for all the extra return spring assembly, so there’s going to be a pretty sharp ledge between your upper and lower.

That still exists along the bottom to some extent, but at the rear, the adapter plate seamlessly blends the upper rail for the MCX upper receiver into the overall look of the gun.

SIG MCX stock adapter
Nick Leghorn for TTAG

In terms of function, this thing feels very solid. It works in terms of adapting the MCX stock to the AR-15 lower receiver, and after some hard use there still isn’t any wiggle. Most importantly, of course, I achieved my initial goal of recreating the rifle I’ve been using in Call of Duty: Modern Warfare in my own gun safe without any additional paperwork.

A quick word of warning lest someone get the wrong idea: this isn’t the only thing you need to do to retrofit your existing AR-15. If you’re using a normal AR operating system in your gun you’re still going to need the buffer tube. But if you have a rifle where everything is contained in the upper (like the MCX) then the MCX stock adapter will work just fine.

Specifications: SIG SAUER MCX Stock Adapter for AR-15 Rifles

Price: $74.99

Overall: * * * * *
It’s a very specific fix for a very specific problem. But if you have that problem, this is definitely the fix.

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    • The SIG MPX operates via a short stroke gas piston system, and does not use a stock with a buffer tube, etc. Hence various stock options.

    • Getting rid of the buffer tube should enable you to run a stock like a Magpul Hunter and enable you to have a NY legal AR without a “protruding pistol grip”. Better than any other workaround like a Thordson stock or ARES SCR lower. Just need Magpul to make one.

  1. Guys, I have EXACTLY this problem. Running the Brownell AR 180 upper (for the AR-15 Lower).

    5.56 AR-180 upper is 16″ rifle (or carbine) barrel, AR-18 and AR-180 style bolt (shortened from its standard) which FITS AR-15 lower. Like the AR-18, this has two springs and two guide rods which fit the bolt and REPLACE the buffer in the buffer tube. The buffer tube only holds in a detent and a spring, it serves no other purpose other than to hold a stock on, if desired (It works as a long pistol if you don’t install a stock) I originally fitted a part suitable for airsoft for a folding stock. It isn’t really ready for the shock a rifle takes in the course of real-world use, and I don’t really want my rifle to have the back end break off and inadvertently discover I’ve converted my rifle to a pistol with busted parts hanging on its back end.

    So those of us running this exotic upper NEED something like this so our stock can fold.
    NOT for someone running anything remotely conventional which relies on the buffer and spring for operation.

  2. KNS has a much better solution for this that only has the rail sticking out the buffer tube hole. I’m using it on my BRN180s pistol setup with a sig folding brace very effectively.

  3. Thanks for the detailed review – generally been very happy with Sig Sauer products! They’re definitely a brand to be trusted. It definitely seems worth purchasing.

  4. The picture with your rifle is not real, the wood grain between the pistol grip and magazine is not the same. Real AR owners rely on posts from people and ther experiences, you should be ashamed.

    • Really? Why on earth are you thinking a plywood background would be regular? All this means is that his lighting is off so the wood grain behind that area of the rifle is not brightly lit enough to show the contrast of the grain. He’s not a portrait photographer, he’s photographing his firearm in his shop. Why makes you so suspicious, anyway? He’s illustrating a firearm, not photographing it for a magazine spread. Why should he be ashamed of his workshop for being what it really is, instead of perfectly regular?

  5. What trigger did you use? I thought I read somewhere that you need a special trigger assembly for it to work with the Sig upper?

    • I just purchased a SIG MCX Rattler upper receiver group in .300 BLK and it shipped with the MCX Stock Adapter for a mil-spec AR lower and the Two-Stage Match Duo AR Trigger Kit.

  6. Kind of curious, but you mention that you Form 1 all of your things. Is there any specific advantage of running SBR over pistol other than having a better stock instead of a brace?
    From what I’ve seen, it looks like there are just more restrictions and more hassle to owning an SBR like crossing state lines, etc.
    I mean, even with a CPL, I can carry a loaded pistol in my vehicle, while you can’t carry a loaded rifle in one.

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