6.8mm Remington SPC  The 6.8mm 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. Based upon the .30 Remington cartridge, it is midway between the 5.56×45mm NATO and 7.62×51mm NATO in bore diameter. It uses the same diameter bullet (usually not the same weight) as the .270 Winchester hunting cartridge. The 6.8mm SPC cartridge was designed to address the deficiencies of the terminal ballistics of the 5.56×45mm NATO cartridge currently in service with the Armed Forces of all NATO aligned countries. The cartridge was the result of the Enhanced Rifle Cartridge program. The 6.8 SPC (6.8×43mm) was initially developed by MSG Steve Holland and Chris Murray, a United States Army Marksmanship Unit gunsmith, to offer superior downrange lethality over the 5.56 NATO/.223 Remington in an M16 pattern service rifle with minimal loss of magazine capacity and a negligible increase in recoil. The goal was to create a cartridge that would bridge the gap between 5.56 mm and 7.62 mm. The program started the design by using a .30 Remington case, which was modified in length to fit into magazines that would be accommodated by the magazine wells of the M16 family of rifles and carbines that are currently in service with the U.S. Armed Forces. In tests comparing various caliber bullets, Holland and Murray determined that a 6.5 mm caliber projectile had the best accuracy and penetration, with historical data going back for decades of US Army exterior and terminal ballistic testing, but a 7 mm projectile had the best terminal performance. The combination of the cartridge case, powder load, and projectile easily outperformed the 7.62×39mm and 5.45×39mm Soviet cartridge, with the new cartridge proving to be about faster than the 7.62 NATO. The resulting cartridge was named the 6.8 Remington Special Purpose Cartridge due to the .270" (6.8 mm) diameter projectile and the fact that it was based on the .30 Remington case. In general, adapting an AR-style rifle to the new cartridge only requires the replacement of the barrel, bolt, magazine & muzzle device (if applicable) of the 5.56 mm-chambered rifle; but to further streamline and simplify the conversion process many parts manufacturers sell complete upper receiver assemblies chambered for 6.8 SPC.