POTD: HK MR 556 A1 Competition Posted March 2, 2015 in AR-15, Photo Of The Day, Rifles with 17 CommentsTags: ar, H&K, MR556 “HKGuns” sent us these gorgeous photos of his Heckler & Koch HK MR556A1 Competition. The Competition models features a new 14″ handguard, a Magpul CTR stock and a very nifty-looking Operators Suppressor Systems (OSS) compensator. Thanks HKGuns! Advertisement