Most factory ammo is produced with the case being minimum dimensions. That is to say the cartridges are dimensioned is such a way that they should chamber and function in the vast majority of commercially produced .223/5.56 chambers.
You can verify this with a case gauge, and you will find virtually all new cartridges will be flush with the “low step” of the headspace indication on the case gauge. Indicating their minimum length for safe headspace.
If you’re feeding a semi-auto this is what you want because the minimum headspace loaded rounds ensure better feeding reliability.
If you’re loading for a bolt action you can full length resize to set the shoulder back .002”-.003” from your fired case shoulder reading. So if you nominally have .006” worth of room to play with for a perfectly in the middle in spec chamber and your minimum headspace factory ammo grows .006” and then you resize the body and set the shoulder back another .002”-.003” you end up with a bit less clearance and less brass expansion. It might help accuracy a smidge too at the cost of not being able to run it in other chambers.
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