6.5 × 55 mm Swedish, also known simply as 6.5×55mm, or in its native military as 6,5 mm patron m/94 (6,5 mm ptr m/94), meaning “6.5 mm cartridge model 1894”, is a first-generation smokeless powder rimless bottlenecked rifle cartridge. It was introduced in the 1890s, and is still one of the most common cartridges in modern rifles built for the Scandinavian market today. The cartridge was developed in a joint Norwegian and Swedish effort starting in 1891 for use in the new service rifles then under consideration by the United Kingdoms of Sweden and Norway. In 1893, the cartridge was standardized and adopted under the name 6.5×55mm to facilitate logistical cooperation between Norway and Sweden. 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 M/1894 rifle, while Sweden adopted the Mauser m/1896 rifle design that was based on a Mauser service rifle designed around the 7×57mm Mauser cartridge.
The 6.5×55mm cartridge has a smaller bullet diameter and lower free recoil than other full-power service rifle cartridges like the 8×57mm, .30-06 Springfield and 7.62×51mm NATO. Thanks in part to its relatively roomy case which was designed for loading long, heavy 6.7 mm bullets, and a 12.2 mm (0.480 in) diameter bolt face, it has proven more successful than other first-generation smokeless-powder military cartridges of similar bullet calibers, 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.
While the original and colloquial cartridge name is 6.5×55mm, there are some variations in chamberings. In addition to the original 1890s specification, three modern chambering and ammunition pressure variations also exist.