To clarify, because mechanical engineers like this sort of thing:
Translational Kinetic Energy = ½ mass * (velocity squared), where mass and velocity units are slugs and ft/sec, respectively. KE will be returned in units of ft-lb.
Linear Momentum = Mass*Velocity , where mass is in slugs and velocity is in ft/sec. Linear Momentum will be in units of slug-ft/sec .
Comparing a few factory loads from Hornady:
For the .308 with 178 grain eld-x, at 300 yards: 2145 ft/sec, 1818 ft-lb, 1.69 slug-ft/sec
For the 6.5 GR with 123 gr sst, at 300 yards: 2090 ft/sec, 1193 ft-lb, 1.14 slug-ft/sec
For the 6 ARC with 105 gr boat tail hollow point, at 300 yards: 2260 ft/sec, 1190 ft-lb, 1.05 slug-ft/sec.
Compared to the 6.5 GR, the .308 has 52% more KE and 48% more linear momentum at 300 yards.
Compared to the 6 ARC, the .308 has 53% more KE and 61% more linear momentum at 300 yards.