The question is whether additional shoulder bumping is really necessary. I've seen several threads recommending bumping at least .002 for semiautomatics. Well ok but won't the FL sizer shape the brass back down to factory new ammo sizes? I must be missing something.
Without gages to measure what is going on after full length resizing, how do you know? Before I purchased cartridge headspace gages I followed the die instructions of sizing to the shell holder plus a quarter turn. After I acquired cartridge headspace gages, none of the ammunition I had sized was correctly dimensioned.
I also have the Hornady headspace comparator gauge. I've measured once fired cases before and after FL sizing and am only seeing -.001 and some with no change at all.
Your Hornady headspace comparator will only provide repeatable results with a bolt gun. It won't work with a semi auto. By the time your hand touches the bolt in a manually operated rifle, barrel pressures are zero. The case has fully expanded and it will be very close to a true chamber size. Whatever the chamber headspace, the cartridge headspace will be the same (within our measuring capabilities) If however you are firing rounds in a gas gun, that Horandy Headspace comparator is useless for setting up your dies. All gas guns open up when there is residual pressure in the barrel, and during extraction the slight residual pressure, which is 650 psia or less, will stretch the case. This little amount of pressure causes the case mouth and shoulder to stick to the chamber during unlock and thus, the base to shoulder dimensions of the extracted case are going to be different from the chamber dimensions.
To accurately size semi auto cases, you need a true gage. I have dropped in my chamber headspace gages in my Wilson cartridge case headspace gages, and they correspond exactly for base to shoulder length. Therefore, it is my opinion that a gas gun owner ought to be using these instead of the comparator, as the comparator is a relative measuring device.
Size the cartridge case so the sized cartridge is between Go and No Go.
If you are sizing for a gas gun, something you have not positively stated one way or another, I recommend pushing the shoulder of the case back about 0.003" from chamber headspace, or if you don't know chamber headspace, just size to the low step. Which corresponds to chamber headspace "Go".