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

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

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Availability: Out of stock

Regular Price: $1,579.99

Special Price $1,329.99

Sig Sauer

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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 upper ships with Sig's flip up sights too!

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

Note: All NFA rules apply.

Specs - More Info
Additional Info
UPC Code 798681587735
Caliber .300 Blackout
Manufacturer Sig Sauer
Sights (detailed) Flip-Up Sights
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
Customer Reviews (2)
The upper I had been waiting for
I have a registered SBR AR lower and wanted an extremely short package. Because of the buffer tube required by most designs, I was stuck with an overall length that I was unhappy with. This upper not only allowed me to remove the dependency of a buffer tube, it provided an upper that functions reliably with the very short barrel. Another benefit was the adjustable piston driven system that keeps the carbon out of the bolt area when firing suppressed. Additionally, I was happy with the extras that came in the kit. Review by William (Posted on 5/27/2019)
Expensive but awesome
I bought this kit a few months back from Osage after shooting my friends rattler. The kit is really nicely done and easy to put together. Definitely heavier than I remembered, but it just feels like quality. Once I had it together, I added a SB tactical folding brace and a Romeo 3. First range trip with my silencerco hybrid mounted had me happier than a kid on Christmas. Shooting this suppressed with both supersonic and subs is amazing. Very mild recoil impulse and accuracy is pretty good depending on the ammo. Overall I would definitely buy it again and think everyone who likes to shoot should have one if these. Review by Zman (Posted on 4/29/2019)
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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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