Check out this discussion of the 5.56x45mm NATO vs .223 Remington and their similarities and differences. Even though many people treat the .223 Remington and 5.56x45mm NATO cartridges as interchangeable rounds, this can be a dangerous practice under the wrong circumstances.
6.5 Grendel. With the addition of the 6.5 Grendel to the product line-up at Alexander Arms® in 2004, the history of no compromise design, engineering, and innovation continues. The 6.5 Grendel provides an extreme range capability for hunting, competition and tactical applications.
The 6.8 mm Remington Special Purpose Cartridge (6.8 SPC, 6.8 SPC II or 6.8×43mm) is a rimless bottlenecked intermediate rifle cartridge that was developed by Remington Arms in collaboration with members of the U.S. Army Marksmanship Unit, United States Special Operations Command to possibly replace the 5.56 NATO cartridge in a Short Barreled Rifle(SBR)/Carbine.