The 6MM-223 cartridge, also known as the 6x45mm, is a wildcat round based on the .223 Remington case. It is modified by necking up the .223 Remington to accept a 6mm (.243 caliber) bullet. This modification increases the cartridge's versatility and performance, allowing it to deliver flatter trajectories and greater energy than the standard .223 Remington.
This cartridge is used primarily in benchrest shooting, varmint hunting, and target shooting due to its improved ballistic performance and manageable recoil. It can effectively be utilized in AR-15 platforms with appropriate barrel changes. The 6MM-223 combines the high velocity and energy retention of the 6mm bullet with the widespread availability and familiarity of the .223 Remington casing.
Reloaders favor the 6MM-223 for its customizability and the ease of forming brass from the readily available .223 Remington. It’s a good choice for medium to small game hunting, including predators and varmints, as well as for competitive shooting disciplines where accuracy and consistency are crucial.
However, as a wildcat cartridge, factory ammunition is generally unavailable, requiring shooters to handload their rounds.