Ted - I have a bullet swage I made, that reduces .376" cast bullets of 250gr. and .306gr., down to .367" for my 9.3x57. They shoot 1/2" larger groups at 100 meters, than the same rifle with jacketed bullets. I am happy. Obviously, sizing a bullet a lot, depends on more than mere thousanths of reduction. .009" in this case. Too, I've run .459" bullets through a .452" die and shot them in my .45 Colt chamberd rifle - that rifle held sub 2" groups at 100 meters with both jacketed and cast. .007" in that case. What this all means is that the way bullets are aligned and sized has more to do with their after accuracy, than how much they are sized.
The only 'secret' for such over-sizing is to lubricate the bullet first.