Originally Posted by
Beetroot
If I were you I'd leave the 3-5 fired cases for now, they probably need annealing so may be giving you measurement issues.
Shoot the 20 new pieces of brass and measure them all after on firing with the Hornady tool.
If they are all measuring the same then set the FL dies up so that it's just setting them back .002" off the fired size.
This should give you the most accurate measurements using what you have at hand.
The idea behind bumping the shoulder is that you do that after ever firing. The older method was to neck size only for 2-4 firings and then FL size before they start get difficult to chamber. But you really want to be avoiding them ever being difficult to chamber, so FL sizing them the bare minimum is the beat way to ensure consistency while not over working the brass.
The 3-5 fired cases probably need trimming, which may be contributing to your difficulty in chambering.
You should trim the cases after resizing.