300 Blackout Upper
Buy 300 Blackout Upper: Sig Sauer Upper Receiver Assembly Sig MCX Rattler 300 AAC Blackout 5.5″ with Tapered Lug M-LOK Handguard Black
$1,499.99 Add to cart

Buy 300 Blackout Upper: Sig Sauer Upper Receiver Assembly Sig MCX Rattler 300 AAC Blackout 5.5″ with Tapered Lug M-LOK Handguard Black


Product Information

Cartridge300 AAC Blackout (7.62x35mm)
Barrel Length5.5 Inches
Barrel FinishBlack
Twist1:5 Inches
Upper Receiver TypeA3 Flat-Top
HandguardM-LOK Handguard
Bolt Carrier Assembly IncludedYes
Country of OriginUnited States of America
Compatible With
Sig Sauer MCX

Delivery Information

Shipping Weight5.350 Pounds

If you can add this item to cart, then it is available in stock.

  • We ship to all states.
  • 100% Money back guarantee.
How long before shipment?

 We typically ship within 3-4 days, 9 days maximum if there is a delay in paper works.

Buy 300 Blackout Upper | Available In Stock

300 Blackout Upper: Rapid deployment, thin, agile, and precise. This is the MCX Rattler. Featuring a 5.5″ PDW barrel, side folding AR buffer tube knuckler. And match duo trigger kit, the MCX platform has never been more compact or accessible.

Ready to go where-ever it’s needed. This upper receiver was designed from the ground up to be as discrete as possible while retaining all the capabilities of the MCX. Free-floating M-LOK handguards matched with a PDW upper, the MCX Rattler upper receiver is the ideal choice for fast-moving operators. 

300 Blackout Upper

Made In United States of America

How to Build a 300 Blackout Upper Step by Step Guide

What you’ll need:

  • Stripped upper receiver
  • Barrel
  • Handguard
  • Flash hider
  • Gas block
  • Upper kit (dust cover and forward assist)

300 Blackout Upper: Installing the Forward Assist

  1. Secure the upper receiver block with a clamp.
  2. Hold the pin with a plier and give it a few taps to get it started. And also drive it into the pin hole at the bottom side of the upper receiver.
  3. Slide the spring over the forward assist and also make sure you position the slanted portion of the forward assist toward the inside of the chamber toward where the bolt would be.
  4. Hold the forward assist in while you drive the pin in. (Tip: use a smaller roll pin punch in the bottom to capture the forward assist while driving the pin in from the top).
  5. Check that the forward assist is installed and functioning properly. You can also consider getting Alumina black to patch up any chips or nicks during the process.

Installing the Dust Cover | 300 Blackout Upper In Stock

  1. Get a tiny C Clamp onto the rod that holds the dust cover in place. Make sure that the C clip is positioned toward muzzle end of the rifle.
  2. Slide the pin right through the hole on the side of the upper receiver and through the front half of the dust cover to hold it in place. However, don’t push it all in. Stop the rod halfway point.
  3. Slide the spring in through the rod. Make sure that the two metal legs are on opposite sides. This step might be a little challenging and may also take a little more time as you adjust them into position. (Tip: Get the shorter tail of the spring facing upwards toward the top of the receiver and the longer tail of the spring with tension facing downwards. Also, you need to make sure that the flat side of the C clamp is facing the flat side of the receiver.).
  4. Push the pin through all the way in to get it installed.
  5. Try flipping the dust cover and check that it opens and captures correctly.

Note: the pin holding the dust cover may possibly slide back down since there’s nothing holding it yet before installing the barrel. 

Installing the Barrel, Hand Guard, and Gas System.

Tip: Before beginning, apply a little bit of weapons grease on the threads of both the upper receiver and barrel extension. This step helps to prevent any seizing in case you need to disassemble the barrel one day in the near future.

  1. Twist the barrel into the upper receiver and make sure that it is secured properly.
  2. Slide the barrel nut over the barrel and also hand tighten it down. Use a torque wrench to continue torquing the barrel nut. (Tip: Typical torque values used for barrels are between 30 and 85 foot pounds ).
  3. Do a quick test fit of the gas block into the barrel to check if there are any imperfections on the inside of the gas block. A 0.75 inside diameter gas block with a low profile is a good choice for pairing with free-floating modular rail design.
  4. Temporarily cinch down the bolts on the gas block to install the gas tube before the final assembly with Loctite.
  5. Test fit the gas tube and gas block into the upper to check if it fits snugly. Once you’re happy with the fit, lightly cinch the bottom bolts before flipping the upper for a closer visual inspection.
  6. Use a bolt carrier into the upper to make sure that the gas tube doesn’t bind up into the bolt carrier.

To Continue Installing the Barrel, Hand Guard, and Gas System

  1. Once everything looks okay during visual inspection, use Loctite on the screws of the bottom of the screws just to make sure that everything is going to be secure. (Tip: Blue Loctite can be used without the need of a blowtorch. It’s also strong enough to keep things from moving and can be broken loose by hand in case you need to go back and change something out later ).
  2. Test fit the handguard into the barrel. (Tip: Be very careful as you advance the handguard into the barrel nut as it will be a very tight fit.) Make sure that there’s not much movement and also not much clearance as you install the handguard. Quick Tip: To quickly and easily check if you have everything properly aligned, you can install a quick detach optic mount to index the upper receiver rails to the handguard rails on top. Make sure to bridge the optic mount between the upper and the handguard rail.
  3. Once you’re happy with the alignment, you can go ahead to cinch down the rail with the mounting screws. To complete the installation, cinch down the two large hex head screws at the back of the upper receiver with an Allen wrench. The screws will go all the way through the barrel nut to prevent any type of rotation and come out through the thread lockers on the other side of the upper receiver. 

Installing the AAC Muzzle Brake

Make sure the ports on the muzzle are indexed horizontally. You can try using coloured shims to get it indexed correctly. Hand tighten the muzzle brake before taking the wrench to push the rest in. 

And you’re done! Now you’ve got a complete .300 Blackout upper all set for the lower receiver.

Frequently Asked Questions

What is a charging handle?

The cocking handle, also known as charging handle or bolt handle, is a device on a firearm which, when manipulated, results in the bolt being pulled to the rear, putting the hammer/striker into a spring-loaded (“cocked”) “ready and set” position, allowing the operator to open the breech and also eject any spent/unwanted.

What is the shortest barrel legal rifle?

A rifle is subject to the NFA only if the rifle has a barrel or barrels of less than 16 inches in length. A weapon made from a rifle is also a firearm subject to the NFA. If the weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length.

What is considered a complete upper?

Stripped vs.
A complete (or assembled) upper also has the ejection port door and the forward assist button factory installed. In fact, a complete upper assembly is the full upper-half of an AR15 rifle. All you would need to fire it would be to attach it to a lower receiver.

Frequently Asked Questions

What is bolt carrier group?

The Full Guide to the AR-15 Bolt Carrier Group - Gun Builders ...

The AR-15 bolt carrier group (called the “BCG)”) is the part of the rifle or pistol that’s responsible for performing semiautomatic fire when the trigger is pulled, with a series of steps in this order. Allowing the firing pin to strike the primer on the chambered round.

What does MIL-SPEC stand for?

Military Specifications

MIL-SPEC is short for Military Specifications. The U. S. military uses standards. Sometimes they conform to say, an ANSI standard, and also sometimes the standard is unique to the military. Or a particular branch of the military.

How are AR-15 pistols legal?

Types of Firearms | USCCA

It requires a citizen to fill out ATF Form 4 and pay a $200 NFA Tax Stamp. An AR-15 pistol allows the shooter to have the size, weight and maneuverability of an SBR but avoids the NFA regulations. An AR pistol is fired like a typical handgun.

What is the short Barrelled rifle rule?

300 Blackout Upper

In January, the ATF decided stabilizing braces will now be considered stocks on pistols, turning them into a short-barrel rifle (SBR), which is illegal unless you register it with the ATF. The new rule allowed for a 120-day period for individuals to register their SBRs, tax-free.


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