The .300 AAC Blackout cartridge was introduced in 2010 by Advanced Armament Corporation. This chambering was initially intended to give the M4 carbine a way to shoot a .30-caliber projectile that was easily suppressed, with none of the drawbacks associated with subsonic 5.56mm (lack of stopping power) or suppressed submachine guns (lack of range, inability to defeat soft armor).
The .300 Blackout is unique in that it is possible to switch from supersonic to subsonic ammunition by merely loading different ammunition, with no adjustments to the gas system required.
Supersonic ammo is on the lighter end, offering enough velocity and energy for use as a hunting or self-defense cartridge. In contrast, subsonic ammunition employs long, heavy bullets particularly suited for use with a suppressor.
Converting your existing .223/5.56 AR-15 to .300 Blackout is reasonably easy. At the very minimum, all you need is a different barrel and an appropriate gas tube.
Typically, however, converting your AR is done by replacing the entire upper receiver assembly. It is easy to attach and detach from the lower receiver and a much quicker process than replacing the barrel and gas tube alone.
Best .300 Blackout AR-15 Uppers
1. Aero Precision M4E1 Assembled Upper Receiver
The Aero Precision M4E1 Assembled Upper Receiver Kit uses Aero’s well-known Enhanced Series Upper Receiver, a departure from standard mil-spec uppers. This upper features a 1-piece design and an 8-screw handguard mounting platform, capable of accepting a free-floating handguard.
This upper receiver is designed from the ground up for ease of use, eliminating most hassles involved in installing and removing AR-15 barrels. Although this is a pre-assembled unit ready for installation on your lower receiver, installing and removing the handguard and barrel is a straightforward process that only requires a barrel nut and a wrench.
The M4E1 upper receiver is forged out of 7075-T6 and precision CNC-machined to provide a tight and snug fit. It is compatible with all mil-spec AR-15 lowers and features a standard M4 feed ramp, increasing carbine and pistol reliability.
Three barrel length configurations are available: 8” and 10” versions for pistol and SBRs, and a 16” version for carbines. Each comes with a railed handguard of appropriate length, and all are fitted with a pistol-length gas system with a low-profile gas block.
The barrels are made of chrome-moly-vanadium steel, featuring an M4 barrel contour and a standard, removable A2 flash hider, which can be replaced with a muzzle device, such as a muzzle brake or a suppressor.
The upper receiver is a flat-top type, and the mated handguard features a top Picatinny rail for installing sights and optics and M-LOK mounting points for other rifle accessories.
This upper receiver kit does not feature a bolt carrier group or a charging handle. However, it should be compatible with those from your existing AR-15. You may also purchase a separate set to turn this upper receiver kit into a ready-to-convert assembly, which can be snapped on and off your lower in a matter of seconds.
2. Brownells BRN-180S .300 Blackout Complete Upper Receiver Assembly
The BRN-180S Complete Upper Receiver Assembly is a unique upper receiver, inspired by the classic aesthetics and controls of the Armalite AR-18 and AR-180.
More than just a regular AR-15 assembly, the BRN-180S upper receiver is a conversion kit for your AR-15. This kit employs a short-stroke gas piston system similar to that of the original AR-18, but dimensionally modified to fit on a mil-spec AR-15 lower, allowing it to use standard AR-15 magazines.
The upper is machined from 7075-T6, a robust type of aircraft-grade aluminum. It features a black hard-coat-anodized finish, protecting the upper from everyday wear-and-tear and giving it a classic, timeless look.
The short-stroke gas piston system runs at lower operating temperatures than a DI action and does not redirect fouling directly into the chamber, making it a little more reliable.
When installed on a mil-spec AR-15 lower, you get a modernized hybrid between the AR-15 and AR-18, which requires no buffer tube to function. This system has the added benefit of making your converted rifle or pistol build compatible with a folding stock or folding brace system.
The .300 Blackout version of the BRN-180S upper features a 10” barrel, ideal for building AR pistols or SBRs. The 1:8 twist rate can effectively stabilize bullets weighing up to 240-grain, which covers most subsonic ammunition available on the market today.
The barrel is threaded and fitted with a retro-inspired 3-prong flash hider, which can be removed to uncover the 5/8-24 thread pitch, compatible with most .30-caliber brakes and suppressors.
3. Radical Firearms AR-15 Upper Receiver Assembly .300 Blackout 16”
Radical Firearms produces affordable, 100% US-made AR-15 firearms and parts, providing customers with quality equivalent to mil-spec or better.
The RF AR-15 Upper Receiver Assembly with 16” .300 Blackout barrel is oriented toward budget builds. This item is an excellent choice if you already purchased an AR-15 and you want to try shooting .300 Blackout without buying a new rifle.
Like most AR-15 upper receiver assemblies, this kit does not come with a bolt carrier group or a charging handle. You can re-use your existing rifle’s BCG and CH or separately purchase a new BCG and charging handle to make the conversion process even faster.
The barrel is 16” long and made of 4150V chrome-molybdenum steel, the same type of steel employed in military-issue M4 carbines and M16 rifles. It employs a heavy barrel contour, which is durable and offers good resistance to the heat generated by sustained fire.
Surrounding the barrel is the Radical Firearms FGS handguard, a 12” handguard featuring a full-length top Picatinny rail, perfectly mated with the flat-top rail on the receiver. Although no sights are provided, there is plenty of rail space to install the back-up iron sights and optics of your choice.
The business end of the barrel is capped with a standard A2 flash hider, mounted on a 5/8-24 thread pitch. Although the provided gas check is non-adjustable, this barrel and upper assembly are suppressor-ready out of the box, allowing you to shoot the subsonic ammunition of your choice without issues.
4. Primary Weapons MK109 Pro Upper Receiver Complete 300 Blackout
The Primary Weapons MK109 Pro Upper Receiver is a complete upper package and conversion kit. It includes an M16-type bolt carrier group, a proprietary 9.75” nitrided barrel, and a Radian Raptor charging handle.
This MK109 complete upper receiver assembly is compatible with any mil-spec lower receiver, converting your firearm to a long-stroke gas piston action. It features a BCG complete with a steel operating rod and piston, similar in principle to an AK.
The upper receiver is a top-quality forged aluminum receiver, ensuring a long service life and high durability. It also notably lacks a forward assist, reducing the total amount of parts.
The 9.75” nitrided .300 Blackout barrel features a 1:8 twist rate and a standard 5/8-24 thread pitch, on which a standard A2 flash hider is pre-installed. Although this flash hider can be replaced with the .30-caliber muzzle devices of your choice, the MK109 upper and barrel are strongly recommended for use with a suppressor.
This system also features an adjustable gas block, letting you adjust the amount of gas your rifle uses to operate the action. Although most shooters find that the default setting is perfectly suitable with or without a suppressor, the option is there if you need to make adjustments and fine-tune your upper receiver to your preferences.
Also included with this assembly is a free-floating M-LOK handguard with top Picatinny rail, which mates almost seamlessly with the flat top rail on the upper receiver. You can install many different optics and sighting devices on the top end, and the M-LOK mounting points on the sides and bottom may accept many other accessories, such as lights, lasers, hand stops, and foregrips.
5. Sons of Liberty Gun Works AR-15 East India Upper Receiver Starter Kit 9″ .300 Blackout
The Sons of Liberty Gun Works Upper East India Receiver Starter Kit is a perfect item for enterprising gun owners who want to customize a barreled upper for their next AR build with the parts of their choice. The East India name refers to the British East India Company, which once possessed a gun foundry near Calcutta, India, in the 19th century.
This starter kit includes a forged 7075-T6 flat-top upper receiver, and the .300 Blackout version of this starter kit is fitted with a 9” medium-profile barrel, finished with a QPQ coating for corrosion and wear resistance.
The barrel twist rate is 1:7, making it ideal for stabilizing heavy subsonic bullets while still giving an adequate performance with supersonic projectiles. The muzzle end is a bare 5/8-24 thread pitch, ready to accept a flash hider, a muzzle brake, or a suppressor.
This kit also includes a low-profile gas block and a steel gas tube with a Melonite finish, ensuring long-lasting performance even when shooting several magazines in a row.
This starter kit intentionally does not come with a handguard or a muzzle device and is not shipped with a charging handle or a BCG. Although you can use this kit to convert an existing AR-15 to .300 Blackout, the SoLGW East India Starter Kit is intended for assembling a new build with the parts of your choice, keeping you in control of customizing your AR pistol or SBR to your specifications.
What to Look for in a .300 Blackout AR-15 Upper Receiver
Upper receivers are not created equal, and 2 different products labeled “upper receiver” may be radically different. There are several terms and features to be aware of to choose the right upper.
Upper receivers are sold as either stripped or barreled kits:
- Stripped upper: A stripped upper is just a piece of metal (typically aluminum) with no parts installed. It is generally only relevant when building an AR-15 from scratch or when looking for a replacement part.
- Barreled upper: A barreled upper is an upper receiver with a barrel, gas tube, handguard, and ejection port cover already installed. These kits are sufficient for nominally converting a rifle. Depending on the package, you may need to purchase additional parts separately, such as a charging handle or sights.
The upper receiver of an AR-15 is typically made of aircraft-grade aluminum, such as 6061-T6 or 7075-T6. However, the manufacturing method may vary depending on the company and the price tag. The 3 most common types are cast, billet, and forged.
- Cast receivers: Cast uppers are formed through die casting: Pouring molten aluminum into a mold. Casting is an inexpensive manufacturing method, well-suited for cheap mass production. Although it offers the least amount of strength (impurities and small inconsistencies in structural integrity), as long as the receivers are cast to the proper specifications, they are suitable for most civilian applications.
- Billet receivers: This term refers to an upper receiver that was CNC-milled from a single, solid piece of aluminum. This manufacturing method is more expensive than casting but has become more popular in the last few decades. It offers a good compromise of structural strength and manufacturing costs.
- Forged receivers: A forged aluminum receiver is hammered into shape by specialized equipment from a single aluminum piece. Because this process reforms the metal to the desired shape instead of cutting it, these receivers possess the greatest strength and density, resulting in higher weight and durability.
One last important feature of the upper receiver is whether it is a flat-top or carry handle model.
- Carry handle receivers: These receivers feature an integrated carry handle and rear sight assembly, patterned after the M16A1 or M16A2 rifles. These receivers are ideal for building clones of classic rifles but limit your choices for installing sights and optics.
- Flat-top receivers: These receivers are patterned after M4/M16A4 receivers and feature no sights, instead including a length of Picatinny rails on which you can attach the optics and backup sights. Installing a handguard with an extended top rail allows you to effectively extend the flat-top rail length, letting you install long optics or multiple accessories, such as an optic and a top-mounted laser sight.
For .300 Blackout builds, virtually all of them feature a 5/8-24 thread pitch, standard on all .30-caliber AR-15s, from .300 Blackout to .308 Winchester. Barrel twist rates are usually 1:7 or 1:8. Although both are serviceable and adequate, there are some minor differences:
- 1:7 twist rate: The rifling makes 1 complete rotation every 7” of barrel. The 1:7 is the faster of the 2 twist rates, ideally suited for stabilizing heavy bullets, such as 220- and 240-grain. This twist rate is the one to get if you’re planning to build a suppressed AR pistol, SBR, or rifle.
- 1:8 twist rate: The rifling makes 1 complete rotation every 8 inches of barrel. The 1:8 is oriented for use with supersonic ammunition. Although they can stabilize heavier projectiles, 1:8 barrels are better suited for supersonic ammunition, particularly between 125- and 150-grains. If you’re planning to build a .300 Blackout rifle for hunting, this is the twist rate to get.
Final Thoughts
There are many options for building the .300 Blackout AR-15 of your dreams. Remember that no matter which upper you choose to purchase, it is always possible to swap parts out later for different makes and models until you find the right combination for your ideal pistol, rifle, or SBR build.
