HK MR556 A4 Short-Stroke Piston 5.56 NATO 16.5" - AR-15 Rifle

A solid 5.56 NATO AR-15 without the fluff: the HK MR556 A4 Short-Stroke Piston 5.56 NATO 16.5" focuses on reliable function, practical configuration, and durable construction in a package ready to use straight from the box.

The HK MR556 A4 Short-Stroke Piston 5.56 NATO 16.5" brings 5.56 NATO capability to a semi-automatic AR-15 rifle, using a direct impingement operating system designed for dependable function in any condition.

Consistent Cycling: The direct impingement system delivers consistent cycling across the standard 5.56 NATO ammunition spectrum without requiring frequent cleaning or special maintenance.

Versatile Configuration: With a 16.5" barrel and standard AR-15 ergonomics, this rifle fits the broadest possible range of users and use cases without compromise in any single area.

Upgrade-Friendly Platform: Everything on this AR-15 is standard — stocks, grips, handguards, triggers, and optic mounts can all be upgraded without specialized tools or gunsmithing knowledge.

Weather-Resistant Finish: The Type III Hardcoat Anodized finish provides lasting protection against corrosion and everyday wear — reliable function from the first range trip to the last.

• Caliber: 5.56 NATO
• Barrel Length: 16.5"
• Barrel Twist: 1:7"
• Gas System: Direct Impingement
• Handguard: Free-Float
• Muzzle Threading: 1/2x28
• Magazine Capacity: 30-Round
• Weight: 5.6lb
• Finish: Type III Hardcoat Anodized

Fully compatible with standard AR-15 lower receivers and STANAG-pattern 5.56 NATO magazines. Works with mil-spec carbine buffer systems, standard lower parts kits, and the broad ecosystem of AR-15 aftermarket accessories.

Machined from 7075 aluminum and coated with Type III Hardcoat Anodized, the receiver set offers durable protection against wear and environmental exposure while maintaining precise fit between components.

Covers the core use cases — home defense, range work, and recreational shooting — without specializing in any single role, making it a reliable all-purpose AR-15.

Heckler and Koch is one of the world's foremost firearms manufacturers, with their short-stroke piston systems and build quality trusted by military and special operations units across the globe.