The Rock River Arms A4 Mid-Length Complete Upper in .450 Bushmaster (brand part no. 450B0592) pairs a 16-inch chrome-moly barrel, cut with a 1:24 twist to stabilize heavy .450 Bushmaster bullets, with a full mid-length gas system for smoother cycling. Rock River Arms builds this as a genuinely complete upper: it ships with a bolt carrier group, charging handle assembly, and a 5-round magazine, so there’s nothing extra to source before it’s ready to run. A 13-inch M-LOK-compatible free-float rail keeps the upper light while leaving room for a full accessory loadout, and the included RRA Operator Brake (5/8-32 thread) helps tame recoil from the big-bore cartridge.

Key Specifications

• Chambered in .450 Bushmaster.
• 16-inch chrome-moly barrel.
• 1:24 twist rate, matched to stabilize heavy .450 Bushmaster bullets.
• Runs a mid-length gas system for smoother cycling.
• 13-inch M-LOK free-float rail.
• Includes an RRA Operator Brake (5/8-32 thread).
• Ships complete with a bolt carrier group and charging handle assembly.
• Includes a 5-round magazine.
• Part of RRA’s Tactical Carbine line.

Frequently Asked Questions

Does this upper come complete, or do I need to source parts? It ships complete with a bolt carrier group, charging handle assembly, and a 5-round magazine.

What twist rate does it use for .450 Bushmaster? A 1:24 twist, matched to stabilize the cartridge’s heavy bullets.

What muzzle device is included? An RRA Operator Brake on a 5/8-32 thread to help manage recoil.

Buy the Rock River Arms A4 Mid-Length Complete Upper, .450 Bushmaster

Drop a ready-to-run, big-bore upper onto your lower for hunting or big-bore capability. Shop more AR upper parts from Rock River Arms and order it from our Lubbock, Texas gun shop for fast, insured shipping.