Velocity and powder conundrum

[Harpoon1]

Groups above 27.4 fell apart quickly, so I did not pursue them.

This is highly controversial but, find two or more groups that have the same relative point of impact, ignore group size for the moment. Ideally these groups will coincide with the lowest ES/SD spread. Think “OCW”, Optimum Charge Weight test. Pick the middle charge weight and adjust the “seating depth” to modulate the group size. The F-Class guys recommend .003” increments for that test, so as not to jump over sweet spots!