To me, 20” seems about perfect. Both of my 308s have a 20” barrel. Never chronographed loads out of my BLR, but my Model 7 pushes 150gr TTSXs to about 2860fps. Very handy in the woods, and still good velocity. Most other cartridges I wouldn’t go below 22” but the 308 seems to be very efficient in shorter barrels than most.