I don't know the rate of twist in my 6.5x55 surplus mauser (sporterized), but it doesnt like the 120 grain bullets with any of the powders I have tried. I have found a good load using the Sierra 100 grain HP; I load it over 42.5 grains of IMR 4064 and a CCI 200 primer. Not sure of the velocity, but it shoots sub MOA at 100 yards. Haven't had a chance to try it on game yet, though. My rifle seems to prefer the 100 grain Sierra, the 129 grain Hornady, and is so-so about any bullet in 140 grains. Good luck and good hunting.