Industry standard (SAAMI) specs for the .308 are 2.490" minimum, 2.810" maximum. As noted however, that's not the whole story.
A heavy bullet set deep will reduce the volume inside the casing and boost pressure.
A heavy bullet set too long may have feeding problems in multi-shot rifles.
Outside of that, good to go. You could, for instance, set a .30 round ball in the neck (the old-timers used to do that with light powder loads for camp meat like grouse). The OAL would be something like 2.2", but it would be OK to fire.