After years of diligently slugging the bore, reading loading manuals, and cast bullet manuals I found the phrase " the best cast bullet resizing swage is the throat of the rifle barrel.
With 30 caliber bores and all manner of cast bullets I have come to the conclusion that I will use the largest diameter bullet that will allow chambering without resistance. In some 30/30 and some 308 caliber rifles this allows me to size to 0.310" or 0.311" rather than 0.309" I have discovered that in 30 caliber rifles sizing a cast bullet to 0.308 will usually result in larger groups than I am happy with. Sizing to 0.309" will reduce group size but give the odd flyer. Sizing to 0.310" and the number of fliers is reduced.
If a 30/30, 308, 0r 30/06 will happily chamber a 0.310" powder coated bullet it will usually give equals size or slightly smaller groups than the same bullets sized to 0.309" (usually). To date larger cast bullets have never given a larger group.
I use NOE cast bullet sizing dies - And I size after powder coating.