The .308 with ball powder was designed as a package, to be near-max with a 20" barrel. Most rifle cartridges have the optimum results with 26" barrels.
FWIW, the degradation in velocity is roughly 50 ft/sec/inch from whatever beginning point is offered in the reloading books. 26" is the standard for published velocities of factory ammo, unless otherwise stated.
Hope this helps, Art