Buy a Hornady cartridge case headspace gauge and measure your fired cases and your resized cases.
Below a fired Lake City case fired in my AR15 carbine, after sizing the case shoulder is bumped back .003.
Different cases will not have the same spring back rate, meaning brass hardness will effect shoulder location after sizing.
I also use a JP Enterprise case gauge made from a chamber finish reamer for a final "plop test" of all reloaded rounds. If the loaded rounds drops all the way into the gauge and falls out when inverted the ammo is good to go.
NOTE, Wilson and Dillon type case gauges do not check case diameter and the JP Enterprise gauge does. Below you can see how far the reversed case drops into the Wilson and Dillon gauges and only check headspace length.
Bottom line, the Hornady gauge checks case headspace length/shoulder bump and the JP Enterprise gauge checks case and neck diameter. And when using mixed brass you will have "mixed" case headspace lengths and may have to bump the shoulder back more than with brass from the same lot.
When full length resizing the case shoulder needs to be pushed back below the red dotted line in the image below. And this is where the Hornady gauge makes life easier for proper shoulder bump and die setup.
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