Any advice would be greatly appreciated?
“In order to control as many variables as possible in developing loads, I measure the headspace of each case (for my rifle)”
Sounds great, Larry Willis makes a case comparator, not a head space gage, the case has a length and the chamber has a head space designation. For your 308 there is a minimum/maximum, the difference is .010”, nice to know. I determine the length of the chamber first, I form then fire, everyone else fires then measures, I measure first then fire, all I have to size cases are presses and dies with shell holders. My opinion, a reloader that understand sizing and controlling the length of the case does not need Larry Willis's case comparator, a reloader with minimum shop skills can make a case comparator with the most fundamental of tools.
”I am not too surprised to see large variations in range brass, but had hoped for better results in cases from the same lot (brand has not been a big factor). Perhaps I'm expecting too much from my equipment.
Any advice would be greatly appreciated”
I have purchases once fired cases from the range, without Larry Willis's case comparator, I have measured and sorted cases in groups based on length, no surprise when checking head stamps they matched. My favorite fired case are those fired in trashy old chambers, because I control the length of the case from the head of the case to its shoulder/datum I can use the long cases to off set the effect of a long chamber. For everything else there is the 280 Remington.
Moving the shoulder forward is a matter of chambering a round and pulling the trigger when fire forming.