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.
So, while I wait for the paperwork to clear, I grabbed the adapter from the SIG SAUER store and set to work.
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).
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.
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
Overall: * * * * *
It’s a very specific fix for a very specific problem. But if you have that problem, this is definitely the fix.