The Crux Handguard is an ultra-light weight M-LOK rail for the MR556 and HK416 and Walther HK416 D145RS. Weighing in at a fraction of the original rail,the CRUX handguard features M-LOK sessions at the 3, 6, and 9 o’clock positions. If you’re the person that lives by the saying ounces equals pounds and pounds equal pain this is the rail for you.