21 September 2022
Precisely what mashburn has said above. Your cast bullet will also drop from the mold with different diameters as alloy changes but in all cases, size the oversize bullet down to whatever final size you determine works best in your rifle. Since I do not compete with these to spare our senior members embarrassment , I size my 30 caliber bullets to .309 and keep the final velocity appropriate to the alloy.
Custom molds can be made to drop the bullet from the mold at final size (309 perhaps) using a specified alloy. They do not have to be sized (reduced in size) but instead go through a size die of 310 to apply lube to them. They could also be pan lubed.
My 45-70 bullets drop from a custom mold at final size in 20:1 alloy.
Short answer: I size to .309 for 30 caliber.