The 6.5×55mm Swedish (Mauser) or 6,5 × 55 SE or 6.5X55 Mauser or 6.5X55 Krag is a rimless bottlenecked rifle cartridge. It was developed in 1891 for use in the new service rifles then under consideration by the United Kingdoms of Sweden and Norway. The two nations had independent armies and consequently the normal procedure at the time was for their respective governments to use the same ammunition and then purchase small arms of their choice. Norway adopted the Krag–Jørgensen rifle, while Sweden adopted a Mauser rifle design.
The 6.5×55mm cartridge has a smaller bullet diameter and lower recoil than other full-power service rifle cartridges like the .30-06 Springfield and 7.92×57mm Mauser, but thanks in part to its relatively roomy case with an uncommon 12.2 mm (0.480 in) diameter bolt face which was designed for loading long, heavy 6.5 mm bullets, has proven more successful than other first-generation smokeless-powder military cartridges of similar caliber such as the 6×60mm US Navy, 6.5×54mm Mannlicher–Schönauer, 6.5×53mmR Dutch Mannlicher, 6.5×52mm Carcano and 6.5×50mm Arisaka. [Wikipedia]