Its just that SD figures in slightly on the trajectory and wind drift. It doesn't matter to me but to some very serous long range varmiters I know, its pretty important. The 45 gr XBT drops 2 inches more at 500 yards than does the 53 Gr XBT when both start at 3700 fps. No big deal as most folks don't shoot varmits at 500 yards. Most rifles and scopes are not capable from the box of hiting prairie dogs at that range with any consistancy. Its alao true that there is a slight difference in BC between the two Barnes bullets.