I had an interesting experience with a .308 a couple of years ago that had a spread of about 18 inches at 100 yards. My neighbor is a big time reloader and he loaded up a variety of different loads using different amounts of different powders, all using 180gr nosler partitions. Got it down to maybe 12 inches, but nothing better. Went to a couple of different gunsmiths, no answers there either. Next we went to different weight bullets and different powders. Long story short at this point, my .308 is "made" for 160-165gr, ANY factory or handload 165 seems perfect. I had quite an education with that gun. Find the grain that works best, then go for the specific. Next, figure out your maximum comfortable distance to shoot, then look at the charts that show the energy out to that distance. That's where you begin and then go for the bullet that holds it mass at that distance and can penetrate.