Bore diameters will tend to vary a little, even between identical guns. When smooth forming or what we call "land riders", .0001" one tenthousandth of an inch can be felt when loading. If too tight, it can get stuck part way down. Too loose and it will fall down. There may only be a couple tenthousandths separating those extremes. We are talking about putting a metal object inside a metal hole We want it snug but still be able to push it down the bore with 10 to 30 pounds of pressure on the ramrod. It is nearing a press fit.
Barnes xpb's, Barnes tez's are .4505". Parker be's are real close to .451".
Hornady sst's, Tc Shockwave, Pittman Accumax or Aeromax are .452"
The adjustable sizing die swages the bullet by pushing it thru a cone.