The .276 Pedersen (7 x 51 mm) round was an experimental 7 mm cartridge developed for the U.S. Army and used in the Pedersen rifle and early versions of what would become the M1 Garand rifle.
Summary
Developed in 1923 in the United States, it was intended to replace the .30-06 Springfield in new semi-automatic rifles and machine guns. When first recommended for adoption, M1 Garand rifles were chambered for the .276 Pedersen, which held ten rounds in its unique en-bloc clips. The .276 Pedersen was a shorter, lighter and lower pressure round than the .30-06, which made the design of an autoloading rifle easier than the long, powerful .30-06. The US Army Chief of Staff Gen. Douglas MacArthur rejected the .276 Pedersen Garand in 1932 after verifying that a .30-06 version was feasible.
The .270 Winchester was developed by Winchester Repeating Arms Company in 1923 and unveiled in 1925 as a chambering for their bolt action Model 54[2]. The cartridge is based upon the .30-06 Springfield, and the case is slightly longer due to the necking down process.
Driving a 130-grain (8.4 g) bullet at approximately 3140 ft/s (957 m/s), (later reduced to 3060 ft/s (933 m/s)) the cartridge demonstrated high performance at the time of its introduction and was marketed as being suitable for long range shooting on most big game. Two additional bullet weights were soon introduced: a 100-grain (6.5 g) hollow point bullet for Varmint shooting, and a 150-grain (9.7 g) bullet for larger deer, elk and moose in Big-game hunting[2]. While not an immediate success, over the succeeding decades and especially in the post WW2 period, the .270 Winchester has attained great popularity among gun owners and hunters, ranking it among the most popular and widely used cartridges worldwide. Internationally, firearms manufacturers now offer this chambering in all firearm varieties: bolt actions, single-shots, lever-actions (such as Browning BLR), Pump-actions (such as Remington 7600), autoloaders (such as Remington 7400) and even a few double rifles.
http://en.wikipedia.org/wiki/.270_Winchester
The 6.8 mm Remington SPC (or 6.8x43mm) is a new rifle cartridge that was developed with collaboration from individual members of US SOCOM.[2] Based upon the .30 Remington cartridge [3], it is midway between the 5.56x45mm NATO and 7.62x39mm in bore diameter and velocity. It is particularly adaptable to current 5.56 mm NATO firearms, the cartridge overall length being comparable.
Though ballistically similar to the 1950s-era .280 British, improved propellant powders allow the 6.8 mm to have a smaller case. The 6.8 mm SPC (Special Purpose Cartridge) has a muzzle velocity of 2,625 feet per second (800 m/s) from a 16 inch (406 mm) barrel using a 115-grain Hornady OTM bullet.[4]
http://en.wikipedia.org/wiki/6.8_mm_Remington_SPC
