The ATI 6 mm ARC rifles feature an 18-inch barrel with a 15-inch M-Lok compatible rail. The barrels have a 1:7-inch twist rate, a 5/8x24 thread pitch, and a flash hider at 5/8x25 pattern. The Mil-Sport Rifle and Omni Hybrid Maxx Rifle both feature an ATI SR-1 Rear Stock, while the Omni Hybrid Maxx model also has a Nano Composite Trigger Kit. A 6 mm ARC (short for Advanced Rifle Cartridge) is a rifle cartridge developed from the 6.5 Grendel round, necked down for 6 mm bullets. A 6 mm ARC bullet will remain supersonic at 1,000 yards and beyond, which makes it ideal for situations which require precise hits at longer ranges. Better still, the 6 mm ARC round is short enough to fit inside the receiver of almost any AR-15 pattern firearm. With an upper or a dedicated rifle in 6 mm ARC, the AR-15 you know so well can now reach out and make on-target hits at 600, 700 or even 1,000 yards. - CALIBER 6MM ARC - MAGAZINE (1) magazine with 10rd capacity - BARREL 18" Phosphate Coated Barrel, 1:7.5 Twist, A2 Flash Hider - RECEIVERS Aluminum Upper and Lower - HANDGUARD 10" Free Float Keymod Rail - STOCK 6 position stock