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AR Lower Receiver Extension Adapter for SIG MCX Upper

Once the SIG SAUER MCX hits the market, you’ll be able to use an MCX upper with a standard AR lower thanks to this receiver extension adapter.


The MCX doesn’t use a traditional operating or “buffer” spring like on a standard AR. Consequently, this adapter takes the place of the receiver extension and is required because the MCX uses dual operating springs which sit above the bolt carrier group. This makes the upper a little taller than a standard version. The Picatinny rail section accepts the various stocks.


15 Responses to “AR Lower Receiver Extension Adapter for SIG MCX Upper”

  1. jbgleason says:

    I would really like to see a pic of that mounted. Having trouble visualizing it assembled.

    • BillC says:

      The pic rail is on the outside. So it would need a receiver extension that attaches to a pic rail. The threaded side obviously doesn’t accept an extension because both this and receiver extensions are threaded on the outside. It would be nice to see it mounted, as you mentioned.

      • SSD says:

        You don’t need a receiver extension because the springs are inside the upper, not in a tubular extension to the rear of the receiver. Think of this as a cap.

        • BillC says:

          Well, yeah.
          I also got to shoot the MCx at TIFF. It was a nice rifle. While the OEM stock is a bit unusual looking. I liked it.

    • SSD says:

      Ok, go take a look at your AR. You may call the receiver extension a buffer tube. See where it attaches to the receiver is threaded? This screws into that hole. You don’t use a “buffer tube” or action spring with the MCX. This will be set in its place. Any stocks you may want to mount will attach via the Picatinny rail.

  2. mike says:

    I’m picking up what you’re putting down. Making the attachment 1913 is interesting and particularly interesting to me because it’s not a proprietary system.

  3. Toby says:

    I just noticed that most of the stock options that the MCX uses are going to be compatible with that of Rock River’s PDR and should be vice versa. That will be nice and I can see this option become more common place in the future with weapons that do not require an are buffer tube system.

    • Toby says:

      Correction, I should of said PDS. I know some one will correct me if I don’t mention it now.

  4. Mandingo says:

    Sig Sauer “Don’t assume that anything we make will work with industry standard accessories*.” *Also, do not assume that assorted versions of the same product from our factory will have matching dimensions.

    • Toby says:

      So what you are saying Mandingo is that it is NOT a mil spec 1913 rail.

      • Mandingo says:

        It’s SIG, you never know.

      • jeff creamer says:

        It is mil-std 1913 as is the top rail of the MCX. (same for M400, SIG516, SIG716, MPX and 556xi. Also, MK25, P220 Combat, P226 Combat, P229 Combat, P320, P250, SP2022 and all railed SIG 1911’s are mil-sted 1913)

  5. Joe says:

    Would a FA MCX be possible with a pre-86 lower?