Best accuracy for a given cartridge typically happens when the pressure curve made by the burning powder's the same repeatable shape each and every time. It needs to change a bit for different bullet weights.
With the .308 Win., best accuracy with varmint bullets at 120 to 130 grains has been with the faster powders such as 4198. For 150's and 155's in competition, 4895 or Varget are about right. With 168's through 190's, 4064 has been the most frequent winner in competition. Heavier bullets from 200 to 220 grains seem to shoot most accurate with 4350.
Other extruded powders at these speeds for those weights should also do well, but those mentioned are what I'b most familiar with.
While slower powders oft times shoot a given bullet faster, it's usually at the expense of accuracy 'cause the pressure curves they make ain't that repeatable. Bullets leave the muzzle at more different angles as their barrel time isnt' all the same. The ideal powder type is the one that makes bullets leave at the same place in the barrel's whip cycle.
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