6 MM ARC Ammo (6×38mm), or 6 mm Advanced Rifle Cartridge for short, is a 6mm (.243) caliber intermediate rifle cartridge introduced by Hornady in 2020, as a low-recoil, high-accuracy long-range cartridge designed for the AR-15 platform.
The cartridge makes use of 6 mm (.243) bullets, which are known for their high ballistic coefficients due to the high sectional density with less drag and better energy retention at extended ranges. With the release in 2020, Hornady also launched three factory loads with 103, 105 and 108 gr (6.7, 6.8 and 7.0 g) bullet weights and advertised G1 ballistic coefficients of .512, .530 and .536 respectively
The 6mm ARC Ammo is a medium-power cartridge comparable to the 6.5mm Grendel, but with better ballistics thanks to the greater muzzle velocity achieved by using lighter 6mm bullets and enough power to make use of the heavy 6mm bullets. Some have stated that it offers comparable ballistics to larger 6mm cartridges like the 6mm Creedmoor and .243 Winchester, but out of a smaller cartridge, however this simply is not true when comparing muzzle velocities between them. What the 6mm ARC does offer though, is almost comparable velocities and ballistic performance, but out of a compact cartridge that has lower felt recoil and fits in the standard AR-15 length actions.