One Horse 300 Blackout Complete Upper Receiver - 7.5"

The One Horse 7.5-inch .300 BLK Complete Upper is a USA-made complete upper receiver group with a 7.5-inch medium profile 4150 barrel, pistol-length gas system, 7-inch M-LOK free-float handguard, nitride BCG, charging handle, and 5/8x24 threaded — built to mil-spec standards and ready to mount on any mil-spec AR-15 lower receiver.

The One Horse 7.5-inch .300 BLK Complete Upper is a mil-spec complete upper receiver group built in the USA, featuring a 7.5-inch medium profile 4150 steel barrel with 1:7 twist, pistol-length gas system, 7-inch M-LOK free-float handguard with steel barrel nut, nitride bolt carrier group, standard charging handle, and 5/8x24 threaded muzzle (1/2x28 thread, 5/8x24 thread). BCG and charging handle included. Ultra-compact configuration optimized for suppressed subsonic .300 Blackout use.

One Horse builds their complete uppers around 4150 steel barrels — a higher carbon-content steel than the 4140 used in many entry-level AR-15 barrels that provides improved wear resistance at the bore and chamber surfaces under sustained firing. Nitride barrel treatment provides a corrosion-resistant, hard surface finish on the bore and exterior without adding measurable thickness, and is comparable to chrome-lining for durability in field conditions. The free-float M-LOK handguard removes the handguard from contact with the barrel, eliminating the accuracy-degrading contact pressure that standard carbine handguards impose on barrel harmonics — practically meaningful for a complete upper intended for accurate work at range. The M-LOK attachment system provides accessory mounting at the 3, 6, and 9 o'clock positions for lights, grips, and accessories without adding bulk for unused rail sections. The steel barrel nut provides a more rigid and durable handguard-to-upper connection than aluminum barrel nuts under the thermal cycling of extended range use. The nitride BCG matches the barrel's corrosion-resistant surface treatment throughout the bolt carrier group — bolt, carrier, gas key, and cam pin all benefit from the reduced friction and corrosion resistance of nitride processing. Built in the USA, ready to drop onto any mil-spec AR-15 lower without modification. The 5/8x24 muzzle thread is the standard thread pitch for .30-caliber suppressors.

Caliber: .300 BLK. Barrel: 7.5-inch, medium profile, 4150 steel, 1:7 twist. Gas: pistol-length. Handguard: 7-inch M-LOK free-float, steel barrel nut. Muzzle: 5/8x24 threaded. BCG: nitride. Charging handle: standard, included. Construction: mil-spec. Origin: Made in USA.

4150 steel barrel provides improved wear resistance over 4140 at higher round counts. Free-float M-LOK handguard eliminates barrel contact for consistent harmonic performance. Nitride BCG and barrel treatment provide corrosion resistance and reduced friction throughout. Steel barrel nut delivers rigid, durable handguard-to-upper connection. Made in the USA, drops onto any mil-spec lower.

The right complete upper for .300 BLK AR-15 builds where the buyer wants a USA-made, mil-spec, free-float M-LOK platform with nitride BCG and charging handle included — 7.5-inch, ready to mount.