PRIMARY WEAPONS AR-15 MK1 MOD 1-M UPPER RECEIVER ASSEMBLY 300 BLACKOUT M-LOK The Primary Weapons Systems MK1 MOD 1-M line of upper receivers feature the PWS long stroke piston system. Internally, this line offers the tried and true system that PWS owners have grown to know and love. Featuring a free-float MLOK handguard, forward assist controls, and a more true to mil-spec design, the MK1 MOD 1-M line is a go-to for not only civilians, but also Law Enforcement. With the fully forged upper, the MK1 MOD 1-M is a no-nonsense tool for when you need a high value option that is relentless and unstoppable. PWS bring back the original MK109 in 300BLK. The 9.75" Mfg: Primary Weapons