In components and dies, it is exactly the saem as the 6mm Remington. The difference is in Remington barrels. Originally the 6mm was known as the .244 Remington, and was marketted as a "varmint caliber", therefor a slower twist for lighter bullets. When the .243 Winchester caught on, Remington saw that shooters were using heavier bullets in the .243 for big game, so they renamed the cartridge, and put a faster twist on the barrel. .244 twist 1:12, 6mm twist 1:9.