Standard .260 load = 43.0 x .80 = 34.4 (over fill Grendel case) You might push a 95 grain bullet in the Grendel as fast as a 140 grain bullet in the .260.
The Grendel is a good cartridge but is designed for the ar15 platform. It does not have the case capacity to compare to a 6.5 C, 6.5 L, or .260 Rem in a bolt gun.