Heckler and Koch (HK USA) | MR556A4 URK | 223 Rem | 5.56 NATO | Matte Black

HK's MR556A4 Upper Receiver Kit gives an existing Mil‑Spec AR‑15 lower the genuine short‑stroke gas piston system and cold‑hammer‑forged, 1:7‑twist barrel of the full MR556 A4, without needing to buy a complete rifle. The closed‑gas system keeps hot carbon fouling and waste gas out of the receiver entirely -⁠- a real, measurable difference from the direct‑impingement uppers most AR owners are used to -⁠- and the free‑floating M‑LOK rail with full‑length top Picatinny gives genuine accessory flexibility. Available in 11‑inch and 16.5‑inch barrel lengths, compatible with most common AR‑15 lowers -⁠- real HK416‑pattern engineering as a drop‑on upgrade.