Since I am apparently one of the few responders who has actually used both the .257 Roberts and 6.5x55 for quite a bit of both big game hunting and long-range varmint shooting, and stated that I never could tell all that much difference, I ran some numbers on the Sierra Ballistics program out to 1000 yards.
The bullets used were the best I've found in each caliber for long-range varminting, the 115-grain Nosler Ballistic Tip .25, and the 140-grain Hornady A-Max 6.5.
Muzzle was assumed to be 3000 with the 115 in the .257, and 2700 with the 140 in the 6.5x55. I've found these to be typical muzzle velocities with safe handloads in both rounds, in 24" barrel.
Both rounds were assumed to be zeroed at 200 yards, with a 5-mph crosswind:
The results for the .257--
Drop at 1000: 305 inches
Wind drift at 1000: 47 inches
Velocity at 1000: 1293 fps
The results for the 6.5x55--
Drop at 1000: 336 inches
Wind drift at 1000: 43 inches
Velocity at 1000: 1332 fps
Gee, no wonder I've never been able to tell all that much difference between the two rounds.
I might also state that despite the 6.5x55 being used for a lot of target shooting, I have never been able to tell any difference in the accuracy of the two rounds. In a factory rifle the accuracy potential will vary with the individual rifle: I have owned factory rifles in both chamberings that are medicre shooters, and factory rifles in both chamberings that shot extremely well.
In a custom rifle both are very accurate. This isn't surprising, since the cases are about as close to each other as they can be without being identical.
As stated in my earlier post, I have never been able to tell any difference in the way either cartridge killed big game, even elk. This is not assumed by looking at energy figures, or other assumptions based on bullet weight, but by actually seeing animals being taken in the field.
