Every few years, somebody gets a chronograph and a rifle with a 30" barrel and goes to choppin', shootin', and measurin'.
(Most published factory data on velocities for rifles is for 26" barrels.)
As a generalization, most of the time, for many cartridges, velocity drops by about 70 feet per second, per inch of barrel less than 26 inches.
Now, having said all that, I have read that the .308 case was designed for a 150-grain bullet, using Winchester Ball Powder. And, all this was supposed to be wonderful in a 19" barrel--allegedly, not much faster in longer barrels. I do not know for sure if the data from Winchester for velocities of the .308 are taken using a 19" barrel. It is my surmise that the .308 might suffer less from shorter-than-26" barrels than, say, a .270 or .30-'06. That is, maybe a 40fps loss instead of a 70fps loss.
For all-round usages, and ranges under 400 yards, I'd guess the velocity loss wouldn't be enough to notice if you went as short as 16". The difference between, say, 2,900fps and 2,700 fps is rather unimportant. (Given BATF's behavior, I'd suggest a 16-1/2" barrel as the minimum length.)
Hope this helps, Art
