{"product_id":"hk-mr556a4-upper-receiver-kit-5-56x45mm-nato-16-5-m-lok-ar-15-m16-m4","title":"HK MR556A4 Upper Receiver Kit 5.56x45mm NATO 16.5\" M-LOK - AR-15\/M16\/M4","description":"\u003cp\u003e\u003cspan style=\"font-size: 14pt;\"\u003eThe \u003cstrong\u003e\u003cspan style=\"background-color: #ffff00;\"\u003eHK MR556A4 Upper Receiver Kit\u003c\/span\u003e\u003c\/strong\u003e is a factory complete \u003cstrong\u003e\u003cspan style=\"background-color: #ffff00;\"\u003e5.56x45mm NATO\u003c\/span\u003e\u003c\/strong\u003e upper assembly from \u003cstrong\u003eHeckler \u0026amp; Koch\u003c\/strong\u003e that drops onto any Mil-Spec \u003cstrong\u003eAR-15, M16, or M4-pattern lower\u003c\/strong\u003e, converting a standard direct-impingement AR into HK's gas piston and pusher rod operating system.\u003c\/span\u003e\u003c\/p\u003e\n\n\u003cp\u003e\u003cspan style=\"font-size: 14pt;\"\u003eAt the heart of this kit is a \u003cstrong\u003e16.5\" barrel\u003c\/strong\u003e with a \u003cstrong\u003e1\/2x28\u003c\/strong\u003e threaded muzzle, married to an \u003cstrong\u003eM-LOK handguard\u003c\/strong\u003e that provides a slim, suppressor-friendly accessory mounting platform. HK's \u003cstrong\u003ematte black anodized\u003c\/strong\u003e finish covers the upper receiver and handguard for hard-use durability.\u003c\/span\u003e\u003c\/p\u003e\n\n\u003cp\u003e\u003cspan style=\"font-size: 14pt;\"\u003eThe MR556A4 ships as a complete drop-on kit including the \u003cstrong\u003eupper receiver assembly\u003c\/strong\u003e, \u003cstrong\u003ebolt\u003c\/strong\u003e, \u003cstrong\u003ebuffer\u003c\/strong\u003e, and \u003cstrong\u003ebuffer spring\u003c\/strong\u003e - everything needed above the lower to run on a standard Mil-Spec lower receiver. An \u003cstrong\u003eadjustable gas system\u003c\/strong\u003e lets the shooter tune cycling for suppressed use or varied ammunition loads, while the gas piston and pusher rod design offers an alternative to traditional direct-impingement AR operation.\u003c\/span\u003e\u003c\/p\u003e\n\n\u003cp\u003e\u003cspan style=\"font-size: 14pt;\"\u003eThis is a \u003cstrong\u003e\u003cspan style=\"background-color: #ffff00;\"\u003enew\u003c\/span\u003e\u003c\/strong\u003e factory HK component - the ideal upgrade for owners of a quality AR-15 lower who want to bring HK's piston reliability to their build.\u003c\/span\u003e\u003c\/p\u003e","brand":"HK","offers":[{"title":"Default Title","offer_id":55470842315074,"sku":"178700","price":2179.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0997\/7921\/5682\/files\/hires_71f942aa-cb72-43be-945c-bcca077d22fa.png?v=1785417438","url":"https:\/\/centerfiresystems.com\/products\/hk-mr556a4-upper-receiver-kit-5-56x45mm-nato-16-5-m-lok-ar-15-m16-m4","provider":"Centerfire Systems","version":"1.0","type":"link"}