G David Tubb reckons that increasing the powder charge by 1 gain will return a 1" higher POI at 100yds. A couple of people I shoot with treat this as a near certainty.
However, while developing loads with my Tikka T3 20" barrel .308 I noticed that changing from 41.5 to 42.5gr of N140 with a 155gr SMK didn't move the POI, it only tightened the group.
How can this be?
A clever chap suggested that the barrel lenght means the extra bit of powder isn't being burnt efficiently - a diminishing return if you will.