GP,there are 2 things I can suggest to lower S.D.,& I can't swear they'll get it as low as you want,but they will help. Go to Federal Bench Rest Primers. They are more consistant. Check & see if you are puting any crimp on your bullet in the case. Wheather you are or are not , it has to be consistant,& that's the nature of our fun & the very air I breath when shooting NRA HI-POWER.
CONSISTANCY
SHOT to SHOT