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SIG SAUER Sig MCX Upper Receiver Assemebly, M-LOK UAMCX-5B-300-BLK-TAP Caliber: .300 AAC Blackout, Barrel Length: 5.5 in, 18% Off w/ Free Shipping

SIG SAUER MCX Rattler Upper Receiver Assembly, .300 BLK, M-LOK Handguard, 5.5 inch, Black, UAMCX-5B-300-BLK-TAP
SIG SAUER MCX Rattler Upper Receiver Assembly, .300 BLK, M-LOK Handguard, 5.5 inch, Black, UAMCX-5B-300-BLK-TAP
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SIG SAUER MCX Rattler Upper Receiver Assembly, .300 BLK, M-LOK Handguard, 5.5 inch, Black, UAMCX-5B-300-BLK-TAP

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Questions & Answers
Does this come with front and rear sights? Expert Answer

These uppers do NOT include the front and rear sights any longer after revisions made by Sig Sauer.

by Armando R., Gear Expert, September 26, 2022
See 1 More Answer
Do the 5 in 300 blackout uppers cycle subsonic and supersonic unsuppressed? If not will the 9inch 300 black cycle both reliably? Expert Answer

The uppers should cycle with either ammunition, but when shooting sub sonic the action will cycle much more efficiently and properly with a suppressor.

by Armando R., Gear Expert, September 4, 2019
View All Q&A You Have a Question? Ask!
Customer Reviews
Helpful Positive Review
Rattler Upper by Jay, Verified Owner, October 8, 2019
I picked up the 5" upper to set up my MCX as a Rattler. It has run flawlessly. I've run my suppressor on it without issue as well. I did switch the gas setting for subsonics when using my OSS can. Super happy with my purchase.
2/2 found this helpful
Helpful Positive Review
Extra Items are Included by JoeThumbs, Verified Owner, December 2, 2019
Firstly, love them or hate them, it’s a nice Sig. There seems to be some confusion on how this conversion kit works, as well as exactly which items are included in this kit. This kit does include all the items required to make this work on a standa...
Read More...
21/21 found this helpful
Helpful Positive Review
Best 300blk upper by Capt Forest, Verified Owner, September 20, 2021
This kit comes with everything you need to convert your lower and attach this upper. Kit comes with a match grade trigger that is guaranteed to work with the upper. Some triggers are finicky with sigs internal buffer system. This also comes with ...
Read More...
3/3 found this helpful
Helpful Negative Review
Poor condition by Diasspointed, Verified Owner, July 26, 2019
I received this upper today. The upper is dirty from use with scuffs, wear. The spot where the rear pin goes through to connect the upper to the lower looks like it was hit on something very hard or possibly with a hammer. I was under the impression ...
Read More...
11/20 found this helpful
Most Recent Reviews
The best short 300 blackout Upper by wombat, Verified Owner, November 29, 2024
Short and and MCX...what's not to like? Extremely reliable, awesome suppressor host, don't settle with a 9mm PCC when you can have a 300AAC in your backpack.
Most Recent Reviews
America#1. by Trapper, Verified Owner, April 26, 2024
Just as labeled. Best price on the net. Very satisfied. Shipped quicker than advertised.
Read All 6 Reviews
Caliber:
.300 AAC Blackout
Barrel Length:
5.5 in
Gas System Length:
Carbine Length
Barrel Twist Rate:
1-5
Color:
Black
Gun Model:
SIG Sauer MCX
Condition:
New

Code: SD9-UFH-MCX-UAMCX-5B-300-BLK-TAP

MPN: UAMCX-5B-300-BLK-TAP

UPC: 798681587735

List: $1,999.99Save 18%
$1,649.00

ATTENTION! We currently do not ship this product to P.O. Boxes, AK, CA, CT, DE, HI, MA, NJ, NY, U.S. Territories, or select Colorado locations. Please see our policies for more information or a detailed list of locations.

Product Info for SIG SAUER Sig MCX Upper Receiver Assemebly, M-LOK

This Sig Sauer Sig MCX Upper Receiver Assembly, M-LOK is a complete upper assembly kit built to improve your weapon's function. These AR15 Upper Receivers crafted by SIG SAUER convert any MIL-SPEC AR Lower into an MCX Rattler with ease. Each Sig Sauer Sig MCX Upper Receiver Assembly - 5.5in 300BLK includes an upper assembly, lower receiver stock adapter kit, side folding AR buffer tube knuckler and a match duo trigger kit. Complete or upgrade your weapon build with this high quality Sig Sauer Sig MCX Rattler Upper Receiver Assembly.image

WARNING: California`s Proposition 65

Specifications for SIG SAUER Sig MCX Upper Receiver Assemebly, M-LOK:

Manufacturer: SIG SAUER
Caliber: .300 AAC Blackout
Barrel Length: 5.5 in
Gas System Length: Carbine Length
Barrel Twist Rate: 1-5
Color: Black
Gun Model: SIG Sauer MCX
Gun Rail: M-LOK
Gun Make: SIG Sauer
Magazine Included: No
Condition: New
Threads per Inch: 5/8x24

Package Contents:

  • SIG SAUER Sig MCX Upper Receiver Assemebly, M-LOK

SIG SAUER Sig MCX Upper Receiver Assemebly, M-LOK Orderable Models

List of Orderable Models

  • SIG SAUER MCX Rattler Upper Receiver Assembly, .300 BLK, M-LOK Handguard, 5.5 inch, Black, UAMCX-5B-300-BLK-TAP , MPN: UAMCX-5B-300-BLK-TAP , UPC: 798681587735 , Code: SD9-UFH-MCX-UAMCX-5B-300-BLK-TAP

SIG SAUER Sig MCX Upper Receiver Assemebly, M-LOK Unavailable & Discontinued Models

List of Unorderable Models

  • SIG SAUER Upper Receiver Assemebly, Sig MCX, .300 BLK, 16in Barrel, M-LOK, UAMCX-16B-300B-TAP , MPN: UAMCX-16B-300B-TAP , UPC: 798681579594 , Code: SD9-UFH-MCX-UAMCX-16B-300B-TAP
  • SIG SAUER Upper Receiver Assemebly, Sig MCX, .300 BLK, 9in Barrel, M-LOK, UAMCX-9B-300B-TAP , MPN: UAMCX-9B-300B-TAP , UPC: 798681579624 , Code: SD9-UFH-MCX-UAMCX-9B-300B-TAP
  • SIG SAUER Upper Receiver Assemebly, Sig MCX, 5.56 NATO, 16in Barrel, M-LOK, UAMCX-16B-556-TAP , MPN: UAMCX-16B-556-TAP , UPC: 798681579600 , Code: SD9-UFH-MCX-UAMCX-16B-556-TAP
  • DEMO, SIG SAUER MCX Rattler Upper Receiver Assembly, .300 BLK, M-LOK Handguard, 5.5 inch, Black, UAMCX-5B-300-BLK-TAP , MPN: UAMCX-5B-300-BLK-TAP-DEMO , Code: SD9-UFH-MCX-UAMCX-5B-300-BLK-TAP-DEMO
  • SIG SAUER Upper Receiver Assemebly, Sig MCX, 5.56 NATO, 11.5in Barrel, M-LOK, UAMCX-11B-556-TAP , MPN: UAMCX-11B-556-TAP , UPC: 798681579587 , Code: SD9-UFH-MCX-UAMCX-11B-556-TAP
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Last updated 2026-07-06 UTC.