The 308 Winchester is based on a 30-06 Springfield case shortened from 63mm (2.494 inches) to 51mm (2.015 inches). It was introduced in 1952, the year of my birth so, like me, is no spring chicken. We must never forget that the 30-06 Springfield is the most powerful cartridge ever adopted by a major military power. The 30-06 Springfield and its 1903 Springfield bolt-action rifle defined what we think of as “standard” action length. In part this was its undoing; its later Garand rifle was long and heavy; we wanted a shorter and more efficient self-loading action.
Well into the Fifties the 30-06 Springfield was not only America’s service cartridge, but also our most popular sporting round. Unabashedly, I am a 30-06 Springfield guy. As such, I have huge experience with the 30-06 Springfield … and not nearly as much with the 308 Win. In performance on game, the 308 Win and 30-06 Springfield are similar. Always it depends on who is doing the loading but, with greater case capacity, with bullets up to 180 grains the 30-06 Springfield averages about 150 fps faster than the 308 Win.
Game animals are unlikely to discern the difference; there’s really nothing you can (or should) do with a 30-06 Springfield that you can’t (or shouldn’t) do with a 308 Win. The gap widens with heavier bullets. These days few of us actually use heavier bullets, but with 200-grain bullets the 30-06 Springfield is about 200 fps fast, and with 220-grain slugs the 30-06 Springfield is nearly 300 fps faster than the 308 Win.