The .257 Roberts, 6.5X55, and a lot of other 6.5s, are just about the greatest all-around cartridges ever. Lot a stopping power at 300m and very good out-of-the-box accuracy, just where the .223 starts losing its allure.
Reports from Afghanistan are that the .223 is making reliable hits fired from sniper weapons at extreme ranges, but that the bad guy is not hurt too badly unless it's a head or chest shot. In other words, they can keep shooting back.
Pete Kokalis theory is that in order to get accuracy from the 5.56, they have increased the rifling, so you get a small bullet drill-through at long range, rather than the bigger bullet penetration and expansion you need to keep the bad guy down for keeps.
As a kid, I bought a surplus, as new Swedish Mauser in 6.5X55. On the range, it displayed phenomenal accuracy. A 1" group at 100yards ... as delivered for $54.95! Still have it 45 years later.
I went a slightly different route, picking up a surplus Swedish M/21 B.A.R. also chambered in the 6,5x55mm cartridge you describe and which so favourable impresses us both. Though the barrel was plugged when obtained, once the 1968 amnesty came around it received a new .30-06 barrel from a M1918A2 BAR, courtesy of a local Air National Guard armorer. But I always wondered how it would do in the original chambering.
About a decade back, I found a like-new Swedish BAR barrel for $75 at a gun show, and restored the old shooter to its original chambering, and to the configuration in which it most likely served some Finnish gunner during the 1939-40 *Winter War* after the Russians invaded Finland in November, 1939. The Finnish *SA in a rectangle* is one clue to that probable use; so are the bloodstain discolourations across one side of the stock and receiver. Some 65 years after it's time of service, it remains a superb instrument.