Re: 6.5x55mm Swede question
It predates the 8x57 by 14 years according to wikipedia. developed in 1891. I don't think there is a parent cartridge, but brass is readily available. I have accidentally resized and fired a .308 cartridge because I wasn't paying attention when separating from the tumbler and saw no ill effects with a light load, but I am not sure I would recommend it. Wikipedia refers to is as a first generation cartridge which to me would suggest it has no parent.