The 6.8mm Remington SPC (6.8x43mm, 6.8 SPC) is an intermediate rifle cartridge designed within the U.S. Army Marksmanship Unit (AMU), SOCOM (U.S. Special Operations Command), and manufactured by Remington. Its parent case is the .30 Remington.
The 6.8 SPC was created to address problems with the 5.56 NATO cartridge that is currently being fielded by U.S. military personnel. The development of the round was created with the collaboration of the U.S. SOCOM troops from the 5th Special Forces Group and elements from the U.S. Army Marksmanship Unit.
The program set off by taking the 6mm PPC cartridge and necking it up to 6.5mm; however, this case was soon discarded due to poor reliability and insufficient magazine capacity when using the STANAG 5.56mm magazine body.
Attention was then brought to the obsolete .30 Remington round. The head and body diameters of this cartridge are larger than 5.56mm, but smaller than 7.62mm, and was chosen due to the fact that the smaller head diameter required less metal to be cut from the AR bolt, which improves the bolt's service life. Rather than use the 5.56mm's rim with the SPC body to keep from modifying the bolt, the full-diameter rim of the .30 Remington was used; while it required modification to the bolt, the full-diameter rim also helped with extraction of the spent case. The case dimensions were then modified and tweaked to fit in the magazines used in the M16 family of rifles. 
The project team then turned their attention to bore size. Bullets ranging in weights from 90 grains to 140 grains, and ranging from 5.56mm to 7.62mm, were tested. 6.5mm bullets were found to shoot the most precisely, and 7mm bullets were found to be the most destructive; however, the 6.8mm bullet displayed nearly the same precision as the 6.5mm, with nearly the same terminal ballistic profile as the 7mm bullet. When the cartridge was necked down to 0.277 inches (6.8mm) and 115-grain bullets were used, the round showed the best combination of accuracy, reliability, and terminal performance for combat engagements of up to 500 meters. The propellant used in the round was tweaked with a goal to produce a muzzle velocity at least 200 feet per second faster than the 7.62x39mm round; the end result produced a muzzle velocity of around 2650 feet per second, exceeding the muzzle velocity of the M43 round from an AK-47 by around 300 feet per second.
The 6.8 SPC cartridge's overall length (OAL) is similar to a 5.56 NATO cartridges, which makes it quite adaptable to 5.56-chambered firearms. As noted, the case head diameter is larger than that of a 5.56 cartridge, which requires modification of the bolt if adapting a firearm normally chambered for 5.56 (e.g., an AR-15).