Everyone has there way of reducing SD's And ES's so all I can do is give you what I do in hopes it will work for you.
If I have a weapon that has high SD's I always start with things that are physical and correctible.
First = I prep all of my cases (Size, trim and weight sort them) to make them as consistant as possible.
Then I weigh all of the bullets (Sometimes you will find 1 or 2 grain differences) and sort them by weight to load them in batches.
Now that the brass and bullets are as close to the same as I can get them I look at the primers and powder combinations (Some powders require hotter primers and depending on the volume of powder, sometimes a milder primer will improve SDs) I found this to be the case on my STW. It produced lower velocities with large rifle primers but better SDs than the magnum primers that increased velocities but also increased SDs.
Poor SDs are an indication of a poor combination of components. When you make a change, only make one change at a time so you can see/read the effect it has on the SDs.
I also look for a powder that has a case density near 100% at maximum pressure. Excessive air
space in a case can change the burn rate of powder. Also look for primer seating consistency this can also have an effect on primer performance.(If you have a primer that seats with little or no effort DISCARD IT.
Checking the performance of your chronograph against another is a good idea also.
Unfortunately there is no magic method/formula for improving SD's so just address one thing at a time and you should find a way to get it into the low teens or better yet single digits.
J E CUSTOM
I followed your advise, weighed the projectiles, added another half a grain to nearly fill the case, reamed the primer pockets and weighed the cases , and guess what It worked. I had two chronographs back to back just to make sure the figures were close to each other and they were.
ES is now in the low teens and I am getting 3107 av M.V. and it grouped in 1/2 inch at a 100 yards.
Thankyou kindly for your most helpful advice .