Heckler and Koch MR556A4 URK Complete Upper Features The Heckler and Koch MR556A4 URK upper receiver kit is chambered for 5.56 NATO and .223 Remington giving builders flexibility in the ammunition they can run through the finished rifle. A 14.5 barrel with a 1:7 twist rate stabilizes a wide range of bullet weights from lighter varmint loads to heavier longer-range projectiles. This barrel length and configuration is intended for an SBR build so buyers should confirm their own NFA paperwork is in order before assembly. Heckler and Koch s reputation for engineering reliable duty-grade rifle platforms carries directly into this upper kit. Finished in matte black the upper receiver presents a clean duty-appropriate appearance that matches typical HK rifle finishes. As a complete upper receiver kit it is designed to mate with a compatible AR-pattern lower giving builders a straightforward path to finishing a rifle around proven HK components rather than sourcing each part individually. The MR556 line has built a strong reputation among shooters who want HK quality in an AR-platform rifle rather than the company s proprietary systems. For builders working on a registered SBR or similar compact configuration the HK MR556A4 URK upper offers a trusted foundation. Heckler and Koch MR556A4 URK Complete Upper Specifications Manufacturer: Heckler and Koch Model: MR556A4 URK Complete Upper Caliber: 5.56 / .223 Rem Barrel Length: 14.5 Twist Rate: 1:7 Color: Black Finish: Matte Black Available At: GrabAGun