far ahead of its time, as the U.S. itself adopted an intermediate cartridge — 5.56×45mm NATO — by the end of the following decade. Soon after America's large-scale involvement in Vietnam commenced in 1965 the 5.56×45mm NATO ArmaLite AR-15 rifle, later standardised as the M16, was purchased in ever increasing numbers and by the late 1960s had displaced the 7.62×51mm NATO M14 rifle in combat units. After insisting on a .30 calibre round with full-power ballistics almost identical to those of the existing .30-06 Springfield, the U.S. then adopted the 5.56×45mm NATO intermediate cartridge, which demonstrated the emergence and dominance of intermediate cartridges on the battlefield (the other notable one being the 7.62×39mm AK-47 round). The adoption of the 7.62×51mm NATO round and the adaptation of the intermediate cartridge CETME (later developed into the Heckler & Koch G3) and FN FAL designs to fire it, produced rifles that were relatively longer and heavier and had greater recoil. The result was weapons that performed well as longer-range semi-automatic rifles, but were more cumbersome and only marginally controllable in automatic fire. These guns also had a higher training burden and were not well suited to soldiers of smaller stature, again due to the recoil. Coincidentally, in 2002 the Americans developed a military calibre intended for the M4 version of the M16 family called the 6.8mm Remington SPC — with similar ballistic properties to the .280 British cartridge — which was intended to provide better ballistics than the 5.56×45mm NATO. In the late 1960s, a version of the .280 British was created using a 6.25 mm bullet in a necked-down .280 British case. It was designed in response to experiments in the U.K. trying to find an ideal military small-arms round. Large caliber bullets were calculated to need more energy to penetrate various levels of body armor to inflict disabling wounds on soldiers. Out of several "optimum solutions" ranging from 4.5 mm to 7 mm, the 6.25 mm was the preferred solution. The bullet had a muzzle velocity of and of muzzle energy. While the 7.62×51mm NATO required of force on impact to penetrate helmets and heavy body armor, the 6.25 mm required only of impact force to deliver the same penetration effects out to 600 m. It remained effective for a longer distance and produced recoil closer to that of the 5.56×45mm NATO. However, it was not designed for very long range