Bob,
Yeah, the 6.5x55 became THE general 6.5 cartridge--the same sort of deal that happens with many other calibers/cartridges.
The 6.5x55 isn't any better than the 6.5x57, in fact it's very close to idential, but a lot more rifles were made for it, thanks to the Norwegian and Swedish armies. The ballistics are basically the same, and the 6.5x55 works equally well in any action with a 3.1 to 3.4 inch magazine. Abundant ammo and brass and lots of rifles = popular.
