This is a complex issue with no easy answer. What matters is bullet penetration, and expansion. Figuring energy doesn't take into account bullet construction. Many modern bullets have made for some pretty effective kills using smaller bullets with far less energy than normally considered possible. A 130 gr copper bullet fired from a 308 @ 3000 fps will out penetrate a 180 gr lead bullet fired from a 300 magnum @ 3000 fps.
While not perfect, the best method I've found to predict performance is to find the minimum speed a particular bullet needs at impact to expand properly. That is the maximum effective range for that gun.
Remember it is the speed at impact that matters, not at the muzzle, and aerodynamic bullets make a huge difference even at moderate ranges. I can take a very aerodynamic bullet, load it in a 308 at only 2500 fps and at 200 yards it is moving faster, with more energy than a round nose bullet fired from a 300 WM at 3000 fps.
A 308 using modern bullets CAN be more effective than a 300 WM loaded with old school bullets. But you can always use the better bullets in the 300 WM and beat 308.
