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6 X 45 (US VERSION) (6mm-223 Remington)

  

Soon after the release of the .223 Remington as a commercial cartridge, shooters began experimenting with the cartridge in an attempt to improve its performance. Several of these experimenters at various time periods necked up the .223 Remington to 6mm as it provided better external ballistic performance over .22 (5.7 mm) caliber bullets. There are several variations of this cartridge, from the standard version of the cartridge which is simply a necked up version of the .223 Remington without any further modifications or improvements made to it, to various versions with improved shoulder angles.