old colonel,
It sounds like you are having a lot of fun, and that is what this game is all about. Since your barrel is 6 groove, just forget everything I said about "V blocks", etc.Often, old barrels are not exactly round,and the measurement of the slug may vary from one side to the other.Digital calipers are great, but because they read to the tenthousnths(.0001),insignificant differences seem big, when they don't mean much.If you just read to the thousanth(.001), things will look better.It is great that your barrels are tighter at the muzzle, than at the chamber.When your rifle was made, the barrels were likely measured and the tightest end oreniented to the muzzle on purpose. Once again, it is not my rifle, but I would not size the bullets to be the "same or less than" the tightest slug.I would size them to fit easily in a fired case.A case not releasing the bullet(ie jammed into the lede) will drive the pressure up,otherwise a little oversized bullet won't.Once it is moving,and moves one bullet length into the barrel, it is "sized".It can't be larger than the barrel, and this seems to improve combustion of the powder.
I wouldn't worry about the caliper being inexpensive, you can "zero" it.In addition to the above discussion of significant digits, you can't expect to get as uniform readings from a soft lead slug as from a steel "standard".You can "squeeze" a lead slug with the caliper, and make it vary several tenthousanths.To make a long story short, I just don't think you are doing much, if anything wrong.
Mike
