Primary Weapon Systems MK118 MOD 2-M 6mm ARC Complete Upper Receiver - 18"

Complete your new build with the PWS MK118 MOD 2-M Complete Upper. This upper receiver delivers the full long-stroke piston system, PicLok handguard, and 18" 6mm ARC precision barrel as a ready-to-run assembly — no additional fitting, no sourcing individual components.

The PWS MK118 MOD 2-M is a complete upper receiver assembly chambered in 6mm ARC, built around PWS's long-stroke piston system and an 18" barrel with a 1:7.5 twist rate, designed for precision shooters building or upgrading a long-range AR-15 platform.

The MK118 MOD 2-M upper delivers a purpose-built combination of precision, reliability, and versatility for demanding long-range applications:

PWS long-stroke piston system — suppressor optimized: The long-stroke piston system minimizes carbon fouling in the upper receiver for cleaner, more consistent cycling over extended strings of fire. The adjustable gas system is specifically tuned to reduce back pressure and gas blowback in suppressed configurations — a critical advantage for shooters running a can on a 6mm ARC build.

18" barrel with 1:7.5 twist for 6mm ARC versatility: The 18" barrel maximizes velocity potential from the efficient 6mm ARC cartridge, while the 1:7.5 twist rate stabilizes the full spectrum of 6mm ARC projectile weights — from lighter, high-velocity loads to heavier long-range bullets. A 5/8-24 threaded muzzle accepts popular 6mm and .308/7.62 muzzle devices.

PWS PicLok handguard — Picatinny and M-LOK combined: The PicLok handguard integrates Picatinny rail sections and M-LOK attachment slots into a single platform, balancing direct-mount rigidity for optics and accessories with the slim M-LOK profile for a comfortable support-hand grip.

• Model: MK118 MOD 2-M
• Caliber: 6mm ARC
• Barrel Length: 18"
• Twist Rate: 1:7.5
• Muzzle Thread: 5/8-24
• Gas System: PWS long-stroke piston — adjustable, suppressor optimized
• Handguard: PWS PicLok (Picatinny and M-LOK combined)
• Charging Handle: Radian Raptor — ambidextrous
• Weight: 5 lbs

Compatible with standard AR-15 mil-spec lower receivers. The 5/8-24 threaded muzzle accepts popular muzzle devices and suppressors. Adjustable gas system accommodates both suppressed and unsuppressed operation without additional tuning components.

The long-stroke piston system and precision-machined ambidextrous receiver are finished for long-term durability and corrosion resistance. PWS's adjustable gas block provides fine-tuned control over cycling pressure across ammunition types and suppressor configurations, extending component service life in high-volume use.

An ideal upper for precision rifle builders, hunters, and tactical shooters seeking extended 6mm ARC capability on an existing AR-15 lower — particularly well-suited to suppressed precision builds where piston-driven reliability and adjustable gas management are priorities.

Primary Weapons Systems is a U.S.-based manufacturer known for their long-stroke piston AR-15 and AR-10 platforms, trusted by military, law enforcement, and precision shooters demanding enhanced reliability over direct impingement alternatives. The MK118 MOD 2-M complete upper is backed by PWS's limited lifetime warranty.