Your reply, I am assuming in the above that by case length, you mean from the head to the shoulder datum line in the middle of the shoulder.
You're dead on, I am using the Hornady Cartridge Headspace Gauge, measuring the shoulder datum.
Your question, what are your exact chamber dimensions and what are your exact brass dimensions before you shoot? Case pressure ring measurements correspond to the SAMMI drawing you included: R-P brass = unfired .505" fireformed .512"; W-W brass = unfired .507" to fireformed of .5125"; Hornady brass = unfired .508" to fireformed .512". So, my thinking is that the chamber dimensions are in accordance with the SAAMI drawing.
I did not want to prejudice any responses by including my over pressure signs in my original post, but I have tried NUMEROUS resized case lengths in .002" increments from fire formed -.002" to as much as .020" back. I have found that I cannot use even the minimum charge loads of H1000 and H4831 without pushed back or blown primers and/or ejector pin blown back and stuck into the bolt head.
I will include the bullets I've tried with those powders if you think that makes a difference.