7x57 Mauser one of the greatest all-around big game cartridges ever, no question. 7 mm has one of the best projectile selections of any caliber too. 120, 139/140, 160, 175 are the main ones I like and use in 7 mm. I've owned 7x57, 7x57R and the 7mm-08 Remington which is nearly a ballistic twin of the 7x57. It takes down deer and hogs very, very well. I'd almost go so far as to say, the 140 grain 7mm does a better job than the 150 grain .308 projectiles at the same velocities.
The .257 Roberts is of course the 7x57 Mauser necked down to .25 caliber. Its a good cartridge but more limited in projectile selection. The 100 grain .25 and 115/117 grain are my choice in the .257 caliber. Although most of my quarter bore killing experience comes from the 117 grain bullet out of the .25-06, I can say for certain, that it is not as good a killer and dosen't make as wide a wound channel as the 140 grain 7mm does and is.
The 6.5x55 is a good cartridge, I don't believe I've ever killed anything with one though, mostly because of rifle selection. I'm not that big a fan of the '96 Mauser and I've just not boght anything else chambered in 6.5x55. I have killed with the .264 Win Mag out of a pre-64 Model 70 Westerner and 6.5-06 out of a custom rifle built on a 1909 Argentine Mauser action though and the 120 grain .264 bullet is a pretty good killer, but, in my opinion, dosen't make as nice a wound as the 140 grain 7mm does.
Bottom line, out of those three, with elk, moose, etc possibly on the table, I'd pick 7x57 Mauser for myself and you should too.
"No free man shall ever be debarred the use of arms."- Thomas Jefferson
Last edited by nate45; February 20, 2013 at 06:34 AM.
Reason: spelling, I never heard of a projrcyiles before. :)