Because of the smaller bullet diameter and the same pressure in the case your maximum velocity will be less than that of the .270 Winchester. The 06 case is probably over bore capacity for the 6.5 mm. The smaller cases of the 6.5x55 and 6.5x57 don't appreciably improve on the ballistics available from the .260 Remington. Nor does the old .256 Newton which is a shortened 06 case. You really have to burn a prodigious amount of powder to get a noticable improvement over the standard 6.5s. The .264 Winchester is the best example. It doesn't get you more than 400 fps improvement in velocity despite burning nearly 50% more powder. That translates into roughly 50 yards out at the end of the trajectory string.