HK MR556A1 5.56 NATO Semi-Auto Rifle, 16.5 in Cold Hammer Forged Heavy Barrel, Short-Stroke Gas Piston, HK FFRS Quad-Rail, Troy Folding Sights, Threaded Barrel, 5 Mags, HK Explorer Case
This HK MR556A1 stands out with its cold hammer forged, heavy-contour barrel, a short-stroke gas piston system, and a free-floated HK FFRS quad-rail featuring a continuous, T-numbered top rail. It is assembled in the United States using a mix of German and American components and carries desirable HK markings, including the HK P proof and two-letter date code "BI." Installed Troy Industries Folding Battle Sights, an HK E1 adjustable stock, and an included HK-branded Explorer hard case with five 30-round magazines round out a complete, ready-to-use package.
Condition
Overall Condition: Very good condition, showing only light, superficial signs of prior handling or use.
Bore Condition: Excellent - Clean, mirror finish, no signs of wear.
Bore Rifling: Excellent Rifling - Sharp lands and grooves.
Specific Condition Notes: Minimal markings from previous handling and use. The bore is bright and clean.
What’s Included
- HK-branded Explorer hard case
- HK MR556A1 operator's manual
- HK folding multitool
- Troy Industries Folding Battle Sights (installed)
- HK polymer rail panels (installed)
- Five 30-round magazines; includes a mix of HK translucent polymer magazines (Made in Germany, HK logo) and steel 5.56x45 magazines
The MR556A1 uses a semi-automatic, short-stroke gas piston system with a rotating bolt for clean, consistent operation. Its 16.5-inch, cold hammer forged barrel is heavy-contour, unlined, and free-floated, paired with a 1:7 RH twist to stabilize a wide range of 5.56 NATO projectiles. The barrel is threaded and fitted with a multi-prong flash hider.
The HK FFRS quad-rail handguard provides a free-float interface with a continuous, T-numbered top rail aligned across the receiver (4–14) and handguard (18–30). Troy Industries Folding Battle Sights are installed front and rear, and HK polymer rail panels are in place for a secure, heat-resistant grip.
Controls and furniture include an ambidextrous safety that can be placed on safe regardless of hammer position, a textured HK pistol grip, and the HK E1 six-position, collapsible stock. The rifle retains core HK details such as the forward assist, case deflector, and dust cover. Length of pull is 11.0 inches.
Markings and construction details include an HK P proof on the upper receiver, a two-letter "BI" date code, and a Heckler & Koch Inc. Columbus GA marking on the magwell. The bolt carrier is marked "HK" and "MR." The rifle is assembled in the U.S. using a mix of German and American components.
Overall length runs approximately 33.90–37.68 inches (collapsed–extended), and weight is about 8.96 pounds with iron sights and the FFRS installed. Capacity is 30+1, and this example includes five 30-round magazines comprising both HK translucent German-made polymer mags and steel 5.56x45 magazines.