The new HK suppressed MR556-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 high performance

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. This Suppressor is marked MR556-SD made stricly for HK Operators, picture of upper with OSS SYSTEM

RIFLE COMES COMPLETE WITH HK MR556 OSS SYSTEM

Link Below
http://www.defensereview.com/operator-suppressor-systemsoss-mission-muzzle-cans-almost-no-back-pressure-less-problems-and-more-hits-on-target/