The 6x45mm cartridge is a rifle cartridge that is essentially a 5.56x45mm NATO case necked up to accept a 6mm (.243 caliber) bullet. It offers improved ballistic performance and energy over the smaller 5.56mm round, particularly in terms of terminal effectiveness and retained energy over longer distances.
It is popular among varmint and small game hunters, as well as for tactical applications where users desire greater stopping power and accuracy. The cartridge is relatively easy to load for those with reloading experience, as it uses the same parent case as the ubiquitous 5.56x45mm NATO. Despite its benefits, it remains less common than other popular rounds like the 6.5 Grendel or the 6mm Remington.