6x45mm
What is it?
The 6x45mm (also known as the 6mm-223 or 6mm/.223 Remington) is a wildcat cartridge created by necking up the standard .223 Remington / 5.56x45mm case to accept 6mm (.243") diameter bullets. While never standardized by SAAMI in the United States, it was officially standardized in Europe by the CIP as the 6 x 45.
History
The cartridge originated around 1965, shortly after the commercial introduction of the .223 Remington and the U.S. military adoption of the 5.56x45mm cartridge. Benchrest shooters and hunters sought to enhance the ballistic efficiency and terminal performance of the compact .223 case by loading heavier 6mm projectiles. It became popular during the late 1960s and 1970s, particularly in benchrest shooting, because it has mild recoil, low powder consumption, extended barrel life, and exceptional inherent accuracy.