.276 Pedersen
The .276 Pedersen (7×51mm) round was an experimental 7 mm cartridge developed for the United States Army. It was used in the Pedersen rifle and early versions of what would become the M1 Garand. 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 U.S. Army Chief of Staff Gen. Douglas MacArthur rejected the .276 Pedersen Garand in 1932 after verifying that a .30-06 version was feasible. History and technical notes. Pedersen's round fired a 0.284-inch (7mm) bullet. Comparable to the contemporary Italian 6.5×52mm (0.268 in) Carcano or the Japanese 6.5mm (0.264 in) Arisaka, it produced velocities of around 2,400 feet per second (730 m/s) with 140 or 150 grain (9.1 or 9.7 g) projectiles. The case was two inches  (51 mm) long with significant taper. Tapered cases simplify the extraction, but require the use of highly curved magazines similar to the Kalashnikov, although for the short magazines of the Pedersen and Garand rifles, this was immaterial. Both waxed and bare cartridges were made for the Pedersen and Garand rifle respectively. An armor-piercing T1 cartridge was developed and presumably a tracer. At the time of its introduction, the .276 Pedersen was a solution to a significant problem. The U.S. Army wanted a general issue autoloading rifle that would fire the .30-06 cartridge, but such a rifle was prohibitively  large with existing designs such as the Browning Automatic Rifle and French Chauchat. A weapon of the same weight as the M1903 needed to fire a smaller cartridge. Pedersen's cartridge was viewed as a compromise as it was underpowered compared to most military rifle cartridges. This decreased recoil energy made possible a reliable, lightweight semi-automatic rifle with existing technology. Despite overcoming these early semi-automatic problems, the Garand was chosen because it did not require the use of lubricated cartridge cases for reliable function. The Garand was originally going to be chambered in the .276 Pedersen, but the  logistics of changing all of the infantry's guns (including machine guns) to a new round was judged cost-prohibitive, so the Garand was chambered in .30-06, removing the need for the new cartridge. Immediately after World War II, British designers introduced a series of intermediate-power 7mm cartridges for a different reason than Pedersen. They sought an answer to the Germans' highly successful 7.92mm "Kurz" and various studies on the matter. The U.S. stuck with .30 caliber mostly out of a desire to have a common cartridge between rifle and machine gun combined with the perceived  necessity for effectiveness out to 2,000 yards. Development of a shorter .30 round specifically for use in an autoloading rifle began after the war, and resulted in the 7.62×51mm NATO, a shorter and slightly lighter round that gave nearly identical ballistics to the .30-06. The British studies on various cartridges culminated in the .280 British cartridge, which shared ballistic similarities to the .276 Pedersen in caliber, bullet weight and velocity. Despite the failure to adopt either the .276 Pedersen or later .280 British, the concept of an intermediate power military cartridge of a 6.5 to 7mm diameter  was far from dead. Shortly after the 7.62mm NATO cartridge was adopted, Armalite submitted their AR-10 for evaluation, the U.S. Army suggested they redesign the gun to fire a .256 caliber projectile. Although this suggestion was fruitless, the Army later engaged in many studies of a 6mm SAW cartridge. They, once again, sought to replace autoloading rifle and machine gun cartridges with one round. Current studies are focused on the 6.8mm Remington SPC and 6.5mm Grendel commercial cartridges although their purpose is to improve on the 5.56×45mm cartridge, not to develop a replacement for the  7.62×51mm NATO as well.