Heckler & Koch has teamed up with Operators Suppressor Systems (OSS) to create the integrally suppressed MR556A1-SD in 5.56mm NATO. In the upcoming July 2014 issue of TACTICAL WEAPONS, author David Bahde runs it through its paces.

Bahde writes, “The basic MR556A1 is an excellent rifle with a proprietary gas piston operating system and a free-floating, cold-hammer-forged barrel with an unlined bore. Designed to function with a wide variety of 5.56mm ammunition, the 16.5-inch barrel has six-groove rifling in a 1-in-7-inch twist rate. HK equips the rifle with a two-stage trigger that allows for great accuracy and controllable rapid-fire strings. The rifle’s takedown pins cannot walk free and ensure a tight fit between the upper and lower receivers. Finally, the HK collapsible buttstock is well suited to target or practical applications and can be locked in place if necessary.

“Designed to accommodate the OSS suppressor system, the new MR556A1-SD features a 14-inch Modular Rail System (MRS) handguard with an octagonal shape. The handguard is held in place with new SW11-type screws, which facilitate easy removal, and KeyMod slots line seven of the eight handguard surfaces for adding lights, lasers and other accessories. A continuous top rail supports scopes, sights and other aiming or night- vision devices. Two integrated quick-detach (QD) sling attachment points are built into the rail just in front of the receiver. Oriented towards the top, they accommodate tactical slings correctly.

“The OSS system for the MR556A1-SD consists of two components. The base unit is the Back Pressure Regulator, or BPR, which adds less than an inch to the barrel. Much of it rests under the MRS handguard, and it provides 18 decibels of sound reduction, rivaling many longer suppressors. It also eliminates 95 percent of the backpressure associated with standard suppressors and reduces reliability issues with a minimal bolt speed increase. In essence, the BPR redirects gases rather than simply capturing them, reducing felt recoil and muzzle rise. The second component of the OSS system is the Signature Reduction Module, or SRM, which, when added to the BPR, provides a sound reduction of more than 140 decibels.”

To learn more, check out the July 2014 issue of TACTICAL WEAPONS, available on newsstands and digitally May 20, 2014. To subscribe, go to

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