beginner case sizing headspace question

[Mustbenice1214]

Hello everyone! I am sizing up some 556 LC brass that I cut for 300 blk. This is my first time reloading. I cut my brass to the proper length and I cleaned it, deprimed, etc. My question is: When using SAC 300 blk headspace comparator and a digital caliper I am getting headspace that seems to be way less than that of SAAMI spec. I compared it to some loaded ammo that is also in the same range, albeit less than SAAMI as well.

My measurements are as follows:

300 blk sig subsonic factory loaded- 1.0395
once fired sig- 1.0455
LC brass 556 cut and ran through RCBS AR small base sizing die- 1.0550

All these are well below SAAMI spec. Is it the SAC comparator? Am I looking for the wrong number? Should I just get a no go gauge and compare between that? Am I overthinking this? For something like plinking ammo does it even matter?

Just trying to load up some subsonic ammo as practice before I move on to some FL sizing in my 6.5 prc. Any help appreciated

 

[SalomonQST99]

For first firing, if the case will fit a case gauge and the round will plunk, then I'd shoot it. After the first firing when the case has formed to the chamber, then it's a good time to measure the headspace and see how much to set the case back.
Think first about COAL and the final round fitting the case gauge. If you have any factory ammo, it can be a good comparison to your reloaded round.

 

[mtang45]

Looks like SAAMI spec is 1.0818". You are fine, the case will fire form to the chamber. Eventually you will probably have to bump the shoulder back because it will be too long. Ideally approximately .002" back from the chamber shoulder is what you want. Factory ammo is frequently way short of SAAMI specs.