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HK MR556A1-SD Rifle and Suppressor Available at Rainier Arms

Posted on August 11th, 2014 by ar15news

Heckler & Koch (HK) and Operator Suppressor Systems Inc. (OSS) have teamed up to bring the ultimate complete package to market. Their pairing has birthed the HK MR556A1-SD, which is a package combination of the amazing HK MR556A1 rifle, the new HK MRS KeyMod rail and the new OSS Suppressor giving you the already proven quality and reliability of the HK MR556A1 rifle with a more comfortable, usable design. This could quite possibly be the coolest thing I’ve seen all year.

HK MR556A1-SD : OSS Suppressor 2 HK MR556A1-SD : OSS Suppressor 2

The new HK suppressed MR556A1-SD is equipped with a cutting edge next generation of suppressor technology from Operators Suppressor Systems Inc. (OSS). It is coupled with HK’s unique new Modular Rail System (MRS) to provide the shooter with a high performance integrated weapon system.

The modular rail system (MRS) for the MR556-SD allows the shooter to place individual rail components and accessories anywhere along the length of the seven free sides of the forearm using the slotted interface. HK has been working with OSS to develop the next generation of advanced suppressors for the HK MR series of rifles and carbines.

These suppressors are comprised of two modules, a Back Pressure Regulator (BPR) and a Signature Reduction Module (SRM)™. The BPR1425™ on the MR556A1-SD captures and controls expanding gases instead of simply trapping the gas with baffles. To further reduce the sound signature of the MR556 and eliminate flash, the OSS SRM4 module can be added to the end of the BPR1425.

HK MR556A1-SD : OSS Suppressor 2HK MR556A1-SD : OSS Suppressor 2

Important Features of the MR556A1-SD MODULAR RAIL SYSTEM (MRS)

  • Repeatable rail segment mounting in any position along the 14 inch forearm (9 inch MRS also available)
  • Self-captured rail segment hardware
  • Two integrated Quick Detachable (QD) sling attachment points just forward of the receiver
  • Optional positionable QD mount attachment
  • Octagon profile allows the OSS suppressor to be installed/removed with the rail in place
  • MRS provides more comfort to the shooter’s support hand


Typical 18 dB reduction with the BPR module installed without SRM Less than an inch of increased weapon length with just the BPR Eliminates up to 95% of the back pressure created by traditional suppressors Attenuates the noise signature without adversely affecting the operating system Redirection of expanding gas within the BPR reduces felt recoil and muzzle rise Greatly reduces weapon fouling Little increase in bolt speed Reduces component wear and overheating common to legacy suppressor designs Reduction in back pressure greatly diminishes toxic gases expelled back at the shooter Better weapon balance with over the barrel mounting of the BPR module Typical sound reductions measured at the shooter’s ear with the BPR and SRM4 installed <139 dB Components are designed to self-tighten during firing to prevent loosening Little to no impact shift transitioning from just BPR to BPR and SRM Typical MRTF (mean rounds to failure) 8000 rounds, semi-automatic Internal parts on both the BPR and SRM can be replaced

The HK MR556A1-SD gives the shooter the same outstanding MR rifle performance with the added benefit of more ergonomic handling and noise comfort of the new MRS rail system and OSS advanced sound suppression.

HK MR556A1-SD : OSS Suppressor 1

Obviously with a suppressor being part of the HK MR556A1-SD package you will have to wait a number of months before your paperwork comes back from the ATF, so the earlier you get in on this the better. Right now Rainier Arms has Serial Numbers on hand for Form 3 Transfers and is one of the only places you can order this kit right now. Check it out at

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