The new Smith and Wesson M&P15R is a rifle chambered in 5.45×39. S&W developed this new AR variant due to the rising costs of .223/5.56 NATO ammunition. The 5.45×39 cartridge was developed for the AK-74 rifle in the early 1970s.
Few guns in made the United States are chambered for it, and consequently, surplus ammo is comparatively cheap. The 5.45×39 is not interchangeable with the 5.56×45 NATO (or .223 Remington) cartridge which is the standard cartridge AR-15 platform rifles.