6.5x55 Swedish ammo is part of the 6.5 x 55 group of cartridges that enjoys a long and storied European history. The 6.5 x 55 Swedish Mauser is one of a pair from that group; the Swedes and Norwegians both developed the same round for their respective armies once the two nations became peacefully independent of each other. The Swedish offering is the name most often mentioned when the 6.5 x 55 cartridge comes up. Workmanship on the guns was renowned and prized. If you can find a clean Swedish Mauser, you have indeed found a treasure.
If you physically compared the .308 (7.62 x 51), the .30-06 (7.62 x 63), and the 6.5 x 55 cartridges, the latter would fall in between the first two in size and dimensions. The 6.5 mm bullet diameter is a misnomer since the bullet's actual diameter is 6.71 mm which translates to .264 inches.
The "Sweet Swede" is a go-to cartridge for many Europeans who use it very effectively on the largest of the European game animals. It is widely used and quite effective for taking moose. Bullets for this round have a wide range of weights from less than 100 grains to over 150 grains. Ballistically, the 6.5 x 55 can come pretty close to matching the .308 cartridge giving it plenty of punch for most although not all North American game. Many American and European ammunition manufacturers offer various commercial loads for the cartridge.