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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.

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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.

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This entry was posted on Monday, February 9th, 2015 at 14:55 and is filed under Black Rifle, weapons. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

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

  1. jbgleason says:
    February 9, 2015 at 15:47

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

    • BillC says:
      February 9, 2015 at 16:43

      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:
        February 9, 2015 at 17:04

        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:
          February 9, 2015 at 21:21

          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:
      February 9, 2015 at 17:03

      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:
    February 9, 2015 at 17:52

    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:
    February 9, 2015 at 17:57

    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:
      February 9, 2015 at 18:06

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

  4. Mandingo says:
    February 9, 2015 at 18:05

    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:
      February 9, 2015 at 18:07

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

      • Mandingo says:
        February 10, 2015 at 00:10

        It’s SIG, you never know.

      • jeff creamer says:
        February 10, 2015 at 21:31

        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:
    February 9, 2015 at 23:12

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