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Commercially by Norma
6mm XC (also known as 6XC) is a rifle cartridge, similar to the 6x47mm Swiss Match, that was developed specifically for NRA High Power match shooting by 11-time NRA High Power National Champion David Tubb. It is touted as being one of the most accurate long-range 6 mm rounds in the world that is designed for repeating rifles.
Originally, the case for this wildcat cartridge had to be formed from .22-250 brass, but it is now commercially available.
6 mm XC Wikipedia
The 6XC is a 1000 yard cartridge, comparable to benchrest calibers such as 6x47mm Swiss Match, 6.5×47mm Lapua and 6 mm/22-250; it fits into cartridge class that exceeds the velocities of benchrest calibers such as 6mm BR Remington, 6mm BRX and 6mm Dasher. The 6XC has claimed several wins in NRA High Power National Championships and a number of NRA Long Range championship events (1000 yard events).7.45 g (115 gr) Hollow Point Boat Tail (HPBT): 3,000 ft/s (910 m/s)