6x45

6x45 refers to a family of wildcat rifle cartridges that use a 6 mm bullet in a case derived from the .223 Remington/5.56×45mm NATO case. In most cases the designation implies a case length near 45 mm, though variants differ in length and shoulder angle. The concept originated in the United States as a way to improve long‑range performance in AR‑15–style rifles without changing the action or magazines.

Most 6x45 variants share the same basic idea: a small-diameter 6 mm bullet seated into a necked-down .223 case. Some versions exist as or are based on Ackley Improved designs to gain case capacity and efficiency. Load development and performance depend on the specific variant, barrel twist, and powder choice. Reloading follows standard procedures for 6 mm cartridges, with careful attention to headspace and safe handling of case volume.

Usage and adoption: The 6x45 is a niche wildcat rather than a standardized cartridge. Factory ammunition is not commercially available; ammunition and chamber dimensions must be produced or verified by reloaders and gunsmiths. It is primarily used by enthusiasts for target shooting and small- to medium-game hunting on AR-15 platforms, or in bolt-action rifles built around the same parent case.

See also: .223 Remington, 6 mm cartridges, wildcat cartridges.