I faced the same dilemma when buying a DPMS LR .308. I ended up with the 24" since it will primarily be a benchrest and prone silhouette gun. But to get to the point, I was at the range the other day next to a guy with a Rem. 700 w/24" bbl and one with the 20" bbl. We ran all three guns through chronographs and the difference amongst the 24" bolt, 24" semi auto, and the 20" bolt was 50-75 fps. This showed me that the convenience of the shorter bbl (especially if you are hunting from a blind) far outweighed the neglible loss of velocity.