I'm another newbie looking for information/advice on reasons for variations in headspace and methods for minimizing it - specifically for .308 Win used with Rem 700 rifle.
I've read the books, I have the tools (but do you ever have enough?), and I'm continuing to research. In one of my early experiences, I was unable to chamber some reloaded ammo in spite of using a Wilson case gauge. I've since bought an Innovative Technologies headspace gauge. There are many choices out there, but this one makes sense for me and, based on subsequent results, I am confident in the measurements.
In order to control as many variables as possible in developing loads, I measure the headspace of each case (for my rifle). I've found that even the same batch of casings can vary as much as 0.005" after resizing. I'm currently using casings with a head to bolt face clearance of 0.0015" - 0.0035" (sorting out those that are undersized), but would like to do better.
I'm using a Dillon 550 press with a Dillon full length sizing die. I upgraded the die to the carbide version which tightened the tolerances some. I've also been experimenting with case lube - type and method of use which has yielded some improvements as well.
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.