Force General Curtis Le May tested the ArmaLite AR-15 and was very impressed with it. He ordered a number of them to replace M2 carbines that were in use by the Air Force. In November of that year, testing at Aberdeen Proving Ground showed the ArmaLite AR-15 failure rate had declined to 2.5/1,000, resulting in the ArmaLite AR-15 being approved for more extensive trials. In 1961, marksmanship testing compared the AR-15 and M-14; 43% of ArmaLite AR-15 shooters achieved Expert, while only 22% of M-14 rifle shooters did. Le May ordered 80,000 rifles. In July 1962, operational testing ended with a recommendation for the adoption of the ArmaLite AR-15 rifle chambered in .223 Remington. In September 1963, the .223 Remington cartridge was officially accepted and named "Cartridge, 5.56 mm ball, M193". The following year, the ArmaLite AR-15 was adopted by the United States Army as the M16 rifle, and it would later become the standard U.S. military rifle. The specification included a Remington-designed bullet and the use of IMR4475 powder, which resulted in a muzzle velocity of 3,250 ft/s and a chamber pressure of 52,000 psi. In the spring of 1962, Remington submitted the specifications of the .223 Remington to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI). In December 1963, Remington introduced its first rifle chambered for .223 Remington a Model 760 rifle. Cartridge dimensions The .223 Remington has a 28.8-grain H2O (1.87 ml) cartridge case capacity. .223 Remington maximum CIP cartridge dimensions. All sizes in millimeters (mm). Americans would define the shoulder angle at alpha/2 = 23 degrees. The common rifling twist rate for this cartridge is 305 mm (1 in 12 in), 6 grooves, Ø lands = , Ø grooves = , land width = and the primer type is small rifle. According to the official CIP rulings, the .223 Remington can handle up to Pmax piezo pressure. In CIP-regulated countries, every rifle cartridge combination has to be proofed at 125% of this maximum CIP pressure to certify for sale to consumers. This means that .223 Remington chambered arms in CIP-regulated countries are currently (2016) proof tested at PE piezo pressure. This is equal to the NATO maximum service pressure guideline for the 5.56×45mm NATO cartridge. The SAAMI pressure limit for the .223 Remington is set at , piezo pressure. Remington submitted .223 Remington specifications to SAAMI in 1964. .223 Remington vs. 5.56×45mm NATO In 1980, the