RRA Complete Mid-Length A4 Upper Receiver 450BM 16" Barrel with 5-Round Magazine

The Rock River Arms complete mid-length A4 upper receiver in .450 Bushmaster is a ready-to-run upper assembly built for shooters who want serious stopping power in an AR platform. Chambered in .450 BM with a 16-inch chrome moly barrel and 1:24 twist rate, this upper arrives as a complete package—bolt carrier group, charging handle, and 5-round magazine included—so you can mount it straight to a compatible lower receiver and get to work. Mid-length gas system tuning keeps recoil manageable and cycling reliable, whether you're hunting medium to large game, running drills at the range, or building a specialized hunting rifle.

The 13-inch free float rail uses M-Lok attachment points, giving you genuine, repeatable accuracy by isolating the barrel from pressure points on the handguard. Rock River Arms paired it with their Operator Brake (5/8-32 thread) to help manage the cartridge's substantial report and muzzle rise. Everything here—the rail, brake, barrel profile, and gas system—is engineered for the recoil signature of .450 Bushmaster, so you get a coherent, function-tested assembly rather than a collection of generic parts.

This is a workhorse upper for hunters, competition shooters, or anyone building a dedicated .450 BM rifle on an AR lower. Rock River Arms has been backing quality AR uppers for two decades, and their reputation for fit and reliability is well-earned in the field.

Key Features

• .450 Bushmaster chambering for high-energy stopping power in an AR platform
• 16-inch chrome moly barrel with 1:24 twist rate
• Mid-length gas system for balanced recoil and reliability
• 13-inch free float rail with M-Lok slots for customization and accuracy
• Rock River Arms Operator Brake (5/8-32 thread) to reduce muzzle rise
• Complete package: BCG, charging handle, and 5-round magazine included
• Drop-in compatibility with standard AR-15 lowers

Specifications

Frequently Asked Questions

Will this upper fit my AR-15 lower receiver?

Yes. This is a standard A4 upper receiver and will drop onto any mil-spec AR-15 lower. Just verify your lower is chambered or will accept a .450 Bushmaster—this upper is caliber-specific.

What comes in the box?

The complete upper assembly includes the bolt carrier group, charging handle assembly, and one 5-round magazine. You'll need to supply the lower receiver and any additional magazines or accessories.

Is the rail free-floating?

Yes. The 13-inch RRA free float rail isolates the barrel from the handguard, which supports accuracy by eliminating pressure points. It uses M-Lok slots for mounting optics, lights, and other gear.

What's the purpose of the Operator Brake?

The RRA Operator Brake helps reduce muzzle rise and recoil impulse, making follow-up shots faster and more controllable—useful for hunting, competition shooting, or extended range sessions with a cartridge as powerful as .450 Bushmaster.