The Rock River Arms A4 Carbine Complete Upper Receiver comes chambered in .350 Legend with a 16" chrome moly barrel featuring a 1:16 twist rate for optimal performance.Rock River Arms A4 Carbine Complete Upper Receiver.350 Legend chambering with 16" chrome moly barrel 1:16 twistRRA 13" M-Lok compatible lightweight free float rail systemRRA Operator Brake with 1/2-36 threadingLow profile gas block with CAR length gas systemComplete with bolt carrier group and charging handle assemblyIncludes one 10-round magazineAnodized finish for durabilityThis complete upper receiver is ready to install on any AR-15 lower receiver making it perfect for hunters and shooters looking for a reliable .350 Legend platform with professional-grade components and accessories included.