260 shares the same bullet diameter as the 6.5x55, but since its a smaller case with usually a slower twist barrel, 140 grain is the max weight usually. I have shot Lapua 155 gr round nose in my custom 260 (1:8 twist) and it was accurate and fast (2550 fps).
6.5x55 is best inside 300 meters, 270 WSM & 7mm Rem Mag would give you another 100m or so of effective range, assuming the shooter can make the shot.