If you have access to a lab balance or at least to a scale with 0.01-grain resolution, try sorting your primers by weight, too. The Courtneys reported this making a difference. Be sure your primers a seated firmly. Ideally, if you took the depth of the primer pocket and subtracted the height of your primer before seating, you would subtract 0.003" from the result to see how far below flush with the case head your seated primer should be. This guarantees 0.003" compression of the anvil into the priming pellet (aka, setting the bridge or reconsolidation), which Naval Ordnance found to be about ideal. Failure to get consistent reconsolidation can lead to small differences in ignition time which can create fliers by giving any motion in the barrel different amounts of time to shift the muzzle position.