One Horse 18" 6mm ARC AR-15 Complete Upper with 15" M-LOK Handguard The One Horse 18 inch 6mm ARC complete upper turns a standard AR-15 lower into a flat-shooting, long-range rifle. It is fully assembled with the bolt carrier group and charging handle installed, so it pins straight onto any mil-spec AR-15 lower receiver. Every upper is assembled, headspaced and test fired before it ships, and it is made in the USA. The 6mm ARC (Advanced Rifle Cartridge) was introduced in 2020 to get more long-range performance from the AR-15 platform. It fires long, high ballistic coefficient bullets that keep a flat trajectory, hold their energy and resist wind drift well past the distances where .223 starts to fade, all with mild recoil. That makes it a strong choice for deer-size game and precision shooting from an AR-15 size rifle. 18" 416R Stainless HBAR Barrel The 18 inch barrel is 416R stainless steel in a heavy HBAR profile for stiffness and consistent shot-to-shot performance as the barrel heats up. A 1:7.5 twist is matched to the long, heavy-for-caliber 6mm bullets the cartridge is built around. The muzzle is threaded 5/8x24 and comes with a muzzle device installed, so you can swap in a brake or suppressor mount that uses the same thread. Rifle-Length Gas System A rifle-length direct-impingement gas system pairs with the 18 inch barrel for a softer, smoother recoil impulse and reliable cycling. 15" Free-Float M-LOK Handguard The 15 inch free-float handguard keeps pressure off the barrel for better consistency and gives you a long M-LOK surface for a bipod, light, sling mounts and other accessories. A continuous top rail runs from the receiver onto the handguard for mounting a scope and night vision or other inline gear. Nitride BCG and Charging Handle Installed An M16-cut bolt carrier group with a black nitride finish comes installed for durable, easy-to-clean operation, along with a standard charging handle. Note that 6mm ARC uses its own bolt face, so use the bolt that ships in this upper. Good Uses for This Upper Deer and hog hunting at longer distances Precision and practical long-range steel shooting Adding a long-range caliber to an AR-15 lower you already own Suppressed shooting with a compatible 6mm or .30 cal suppressor One Horse 18" 6mm ARC Complete Upper Specifications Manufacturer: One Horse Part Number: OH-CU09 UPC: 850078799852 Caliber: 6mm ARC Barrel Length: 18 in. Barrel Profile: HBAR Barrel Material: 416R stainless steel Twist Rate: 1:7.5 Muzzle Thread: 5/8x24 Gas System: Rifle length, direct impingement Handguard: 15 in. free-float M-LOK Bolt Carrier Group: M16 black nitride, installed Charging Handle: Standard, installed Fits: Mil-spec AR-15 lower receivers Test Fired: Yes Country of Origin: USA Related Items Looking for something shorter in 5.56? See the One Horse 10.5" 5.56 NATO complete upper or the One Horse 16" 5.56 NATO complete upper. Frequently Asked Questions Will this upper fit my lower? Yes. It is built for any standard mil-spec AR-15 lower receiver. Do I need special magazines for 6mm ARC? Yes. 6mm ARC needs magazines made for the cartridge, such as 6.5 Grendel or dedicated 6mm ARC magazines. Standard 5.56 magazines will not feed it reliably. Does it come with a BCG and charging handle? Yes. An M16 black nitride bolt carrier group and a standard charging handle come installed. What is the muzzle thread? The barrel is threaded 5/8x24. What twist rate does the barrel use? It uses a 1:7.5 twist, suited to the long, high ballistic coefficient bullets 6mm ARC is known for. Do I need an FFL to buy a complete upper? No. A complete upper is not the serialized part of an AR-15, so it can ship directly to you in most areas. Check your state and local laws before ordering. Is there a warranty? Yes. One Horse backs this upper with a limited lifetime warranty.