The numbers in 6mm and 6.5 millimeter cartridge names say how wide the bullet is, not what the case looks like. That is where most mistakes start. 1. Treating all 6mm cartridges as one The 6mm Remington, the 6mm Creedmoor and the 6mm ARC all fire a 6mm bullet from entirely different cases, and none can be fired in a rifle chambered for another. The 6mm Remington is at 6mm remington ammo. The...