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Sig Sauer MCX Rattler AR15 Conversion Kit - 5.5" .300 Blackout

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Availability: In stock


Quick Overview

Sig Sauer MCX Rattler AR15 Conversion Kit - 5.5" .300 Blackout


This is a new OEM Sig Sauer MCX Conversion Kit that includes an MCX upper receiver, adapters to convert a standard Mil-Spec AR15 lower receiver to accept the MCX upper receiver, and a Sig Sauer two-stage trigger. The trigger installation is required to ensure proper operation of the MCX upper receiver. Using your existing trigger is not recommended and may lead to malfunction.

Sig's MCX conversion upper receivers include the necessary parts to install on an existing AR lower. That means you can essentially build your own pistol or SBR or convert an existing lower into a Rattler.

The kit includes the complete upper with 5.5" .300 Blackout barrel, bolt carrier group, buffer tube eliminator / 1913 back (for mounting a brace or stock), 1913 side folding adapter (for mounting a buffer tube for brace or stock), Sig's Match Duo 2 stage trigger, and a 1913 QD adapter.

The Rattler has a two position gas valve for running suppressed and unsuppressed.

Note: All NFA rules apply.

Additional Information

UPC Code 798681587735
Caliber .300 Blackout
Manufacturer Sig Sauer
Frame Finish Black
Barrel Length (in) 5.5 in
Accessory Rail Yes
Warranty Warranty provided by Sig Sauer. Warranty information is available for Sig Sauer products at

Product Questions

Product Questions

Does this kit come with a bcg?

Referenced product: UAMCX-5B-300-BLK-TAP

The MCX Rattler upper receiver does include the bolt carrier group. 

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How soon can you get these back in stock?

Referenced product: UAMCX-5B-300-BLK-TAP

Response Date: 2-7-19

Sig has been a little slow on delivery but we hope to have these back in stock within the next month or so.

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Do you have to switch triggers if it is already an MCX Virtus?

Referenced product: UAMCX-5B-300-BLK-TAP

Response Date: Jul 2, 2019

You do not have to swap the trigger if you already have a MCX Virtus.

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Customer Reviews

Awesome Review by Andrew
Fits perfectly on my pistol lower. Functions flawlessly. (Posted on 11/20/2019)
Excellent. Although missing a few pieces Review by David
Excellent price on a hard to find price. Everything installed properly and easily. However no Sig flip up sights were included despite being advertised above. Also, the hammer spring was incorrect. Fortunately I had a spare but something to be cognizant of. So, no flip up sights, improper hammer spring, but other than that, excellent. (Posted on 8/19/2019)

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