300 Blackout Uppers – Complete AR-15 300 BLK Upper Assemblies

How to Choose a 300 Blackout Upper

Enhance the performance of your rifle with the 300 Blackout Upper series from Moriarti Armaments. Shop 300 Blackout upper assemblies in multiple barrel lengths, including 5", 7.5", 10.5", 14.5", and 16" configurations, with options for compact pistols, suppressed platforms, hunting, and full-size AR-15 builds. Regardless of your preferred configuration, whether you fancy pistol or carbine length, our 300 BLK uppers offer unparalleled precision and reliability. 

Why choose a .300 Blackout upper?

A .300 Blackout upper offers exceptional versatility for AR-15 owners who want superior performance from short-barreled rifles and pistols while maintaining compatibility with a standard AR-15 lower receiver. Whether you're building a compact home-defense firearm, a suppressor-ready setup, or a hunting rifle, .300 Blackout delivers excellent stopping power with both subsonic and supersonic ammunition. Moriarti Armaments offers complete .300 Blackout uppers in multiple barrel lengths and configurations to suit every shooter.

How to choose a 300 Blackout upper

Barrel length

• 4.5"–7.5" uppers: the most compact Micro ARP and AR pistol builds. 300 BLK burns its powder fast, so short barrels give up less than 5.56 does, and they make great suppressor hosts for subsonic ammo. See Micro ARP 300 Blackout pistols.
• 10.5" uppers: a popular balance of handling and supersonic velocity for home defense and range use.
• 14.5"–16" uppers: full rifle length for hunting and the most supersonic performance; 16" uppers need no short-barrel registration.

Muzzle device and suppressor

Most uppers are threaded 5/8x24, the standard .30 caliber pitch. Choose a flash can, shroud or sound forwarder to push blast forward, a brake to cut recoil, or a mount for your 300 Blackout suppressor.

Twist rate and gas system

Most of our 300 BLK barrels use a 1:7 twist to stabilize heavy subsonic bullets, with a few 1:8 options. Short uppers use pistol or micro-length gas for reliable cycling with both subsonic and supersonic loads.

300 Blackout upper FAQ

Does a 300 Blackout upper use a standard AR-15 BCG and magazines?

Yes. 300 BLK runs a standard 5.56/.223 bolt carrier group and AR-15 magazines and fits any mil-spec AR-15 lower; only the barrel changes.

Can I mix 300 Blackout and 5.56 ammo?

No. A 300 BLK round can chamber in a 5.56 rifle and firing it can destroy the gun. Mark your 300 BLK magazines and uppers clearly.

Learn more: 5.56 NATO vs 300 Blackout · 9mm PCC vs 300 BLK · 10mm vs 300 Blackout. Related: 300 Blackout build kits · all AR-15 upper assemblies