{"product_id":"heckler-koch-mr556a4-complete-upper-receiver-kit-16-5in-5-56-nato-piston-system-51000384","title":"Heckler \u0026 Koch MR556A4 Complete Upper Receiver Kit, 16.5in 5.56 NATO, Piston System 51000384","description":"\u003ch2\u003eProduct Description\u003c\/h2\u003e\u003cp\u003eThe Heckler \u0026amp; Koch MR556A4 Upper Receiver Kit is an updated complete upper receiver and barrel assembly featuring the A4 improvements, including a new barrel profile, adjustable gas block, and updated handguard. Built to retrofit many Mil-Spec 5.56mm AR-style rifles, it replaces the traditional direct-impingement gas tube with HK’s short-stroke piston and pusher-rod operating system. This closed system helps reduce receiver fouling from hot carbon and waste gases, supporting reliable cycling and consistent performance across a wide range of quality 5.56mm\/.223 ammunition. The MR556 A4 upper is assembled in the USA using US- and German-made components and incorporates design elements originally developed for the HK416 platform.\u003c\/p\u003e\u003ch2\u003eProduct Highlights\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003eHK short-stroke piston and pusher-rod system designed to reduce carbon fouling vs. direct-impingement AR-style systems\u003c\/li\u003e\n\u003cli\u003eAdjustable gas regulator for suppressed and unsuppressed shooting configurations\u003c\/li\u003e\n\u003cli\u003eSuppressor-ready concentric 1\/2x28 muzzle threads\u003c\/li\u003e\n\u003cli\u003eM-LOK handguard interface with integral ambidextrous quick-detach (QD) sling cups\u003c\/li\u003e\n\u003cli\u003eContinuous Picatinny top rail interface between the upper receiver and handguard for optics and accessories\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eSpecs\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003eBrand: Heckler \u0026amp; Koch\u003c\/li\u003e\n\u003cli\u003eModel: MR556A4\u003c\/li\u003e\n\u003cli\u003eProduct Type: Complete Upper Receiver Kit \/ Upper Receiver Assembly\u003c\/li\u003e\n\u003cli\u003eCaliber: 5.56 NATO \/ .223 Remington\u003c\/li\u003e\n\u003cli\u003eBarrel Length: 16.5 in\u003c\/li\u003e\n\u003cli\u003eTwist Rate: 1:7\u003c\/li\u003e\n\u003cli\u003eMuzzle Threads: 1\/2x28\u003c\/li\u003e\n\u003cli\u003eGas System: Short-stroke piston with adjustable gas regulator\u003c\/li\u003e\n\u003cli\u003eHandguard Interface: M-LOK\u003c\/li\u003e\n\u003cli\u003eTop Rail: Continuous Picatinny\u003c\/li\u003e\n\u003cli\u003eMPN: 51000384\u003c\/li\u003e\n\u003cli\u003eUPC: 642230270213\u003c\/li\u003e\n\u003cli\u003eAssembly: Assembled in USA with US and German-made components\u003c\/li\u003e\n\u003c\/ul\u003e\u003cp\u003e\u003cem\u003eDisclaimer: We strive to provide accurate and up-to-date product descriptions, specifications, and images; however, errors or omissions may occasionally occur. We are not responsible for any inaccuracies or discrepancies in the content provided. If you notice anything incorrect or unclear, please contact us so we can correct it. When in doubt, we recommend verifying details directly with the product’s manufacturer.\u003c\/em\u003e\u003c\/p\u003e","brand":"Heckler \u0026 Koch","offers":[{"title":"Default Title","offer_id":44077448396922,"sku":"FLX-HK51000384","price":2225.98,"currency_code":"USD","in_stock":false}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/1679\/0365\/files\/283688888.jpg?v=1767677681","url":"https:\/\/tlooutdoors.com\/products\/heckler-koch-mr556a4-complete-upper-receiver-kit-16-5in-5-56-nato-piston-system-51000384","provider":"TLO Outdoors","version":"1.0","type":"link"}