Resizing Cases For Optimal Headspace and Chambering
If you want to monitor your case stretch from head to shoulder (headspace) for optimal resizing invest in an RCBS Precision Mic to measure the before/after sizing headspace (RCBS # 88304 for the .223) MSRP $64.95.
There is a SAMMI headspace range [maximum and minimum] established for all standard caliber chambers, and this gauge will measure how much you have set cartridge headspace back in resizing, ideally 1-2 thousandths maximum, to most closely approximate the headspace of your chamber while allowing the case dimension to chamber but have minimal "slop" (movement).
You can [by error] set the shoulder back too much i.e. under the minimum SAMMI range, and use of the Precision Mic will prevent that from happening (if read correctly) and will prevent the subsequent case separation above the head caused my going belowthe minimum SAAMI headspace limit.
An article on this issue and use of the RCBS Mic gauge in the March, 2012 Shooting Times, although it could have been written more clearly.
Hornady also sells a similar tool (a bit lower cost than the RCBS Mic) that works with the Stony Point system that mounts on your dial caliper.
With either tool, you can precisely measure your shoulder movement as you adjust your dies to obtain the setting you want i.e. 1-2 thousandths shoulder set-back.
All the above presumes your rifle's chamber was machined to SAMMI specs i.e. correctly to start with.