Remington's Adaptive Combat Rifle - the Poerfull ACR 6.5x55mm with Interchangeable Caliber Video 1: ACR Rifle
It is a well known fact, that the 5.56x45mm Caliber has more range & accuracy than the 7.62x51mm; but does not have the stopping power. On the other hand, the 7.62x51mm round has the punch (stopping power); but lacks the range and accuracy in the round. As a result, the new 6.5mm and 6.8mm caliber ammunitions were developed that has the range and accuracu of the 5.56x45mm round and the knock-out punch of the 7.62x51mm Nato round. These new ammunitions have been incorporated into a few modern Assault Rifles.