No two resizing dies are the same, you could have several dies in the same caliber and each one would bump the shoulder back a different amount, we also live in a plus and minus manufacturing world. Meaning you might have a "plus" resizing die and a "plus" shell holder, the simple fix for this is to lap the top shell holder on some sandpaper and remove a few thousandths. You also have military brass "fire formed" in a different chamber and when this brass is resized it tries to "spring back" to its original shape. You can try and adjust your dies to make hard contact with the shell holder with the press reaching cam over and pausing at the top of the ram stroke. Pausing at the top of the ram stroke for three seconds or more reduces the amount of brass spring back. This will also give you more uniform and consistent shoulder bump measurements, pausing lets the brass know who is the boss and to stay put after sizing.
Second you can try small base dies, every piece of once fired military brass I get my hands on is resized once with a small base die to bring the brass back to minimum dimensions. This is because military chambers are larger in diameter and longer in headspace than standard SAAMI chambers. But remember a small base die is "NOT" shorter in cartridge headspace length and is only smaller in base diameter and sizes further down the case. Meaning you need to find out if your cases are too fat or too long and work from there.
Third, get the Hornady cartridge case headspace gauge, it fits all calibers and you only need to buy "ONE" case gauge and not one for each caliber. Until you get a micrometer and vernier calipers you will not have accurate means of checking your fired and resized cases.
Below for comparison are .308 and a 7.62 Nato chamber measurements for a M14 rifle, and remember a machine gun will be even larger in diameter and longer in head space and this effects the spring back rate when resized.
If you want uniformity and accuracy then buy one brand of brass and only fire it in "YOUR" chamber, each brand/type of brass will have different internal case capacities and mixed brass will give you "mixed" accuracy results. If your going to use once fired military brass or range pickup brass then you will have "mixed" resizing results and varying internal case capacities. (Meaning good for short range blasting ammo in rifles like the AR15)
Below a fired case from my AR15 carbine.
And below is the same case that has been full length resized with .003 shoulder bump.
Get yourself a set of vernier calipers and the Hornady gauge, and stop guessing at your problems.
We live in a plus and minus world and each of the .223/5.56 dies below will bump the shoulder of the case back a different amount and squeeze the case to a different diameter after sizing. When you start using mixed once fired brass then all bets are off on what you will end up with.
My Wilson case gauges were retired after getting the Hornady gauge.
I only use them now for paper weights and pen holders. (engaging humor)
