The HK USA MR556A4 Upper Kit delivers a complete 11-inch piston-driven upper receiver assembly chambered in 5.56mm NATO.
- Type: Complete upper receiver assembly
- Caliber: 5.56mm NATO
- Barrel Length: 11 inches
- Operating System: Gas piston (HK416 system)
- Barrel: Cold hammer-forged, chrome-lined, 1:7 twist
- Compatible With: MR556 lower receivers, AR-15 pattern lowers
- Includes: Upper receiver, bolt carrier group, charging handle, rail system
- Use: SBR or pistol build (NFA compliance required)
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