Given the small and awkward 'shoulder' of the 300 Blackout case, I wanted a sanity check to ensure my measurements are sound.  I think they are, just want a second opinion

I'm testing headspace using the Hornady Comparator set with the "B" insert

Factory S&B 147gr FMJ - Pulled 5 rounds from a factory box and they all measured/rounded to 1.059"

I picked through some fire formed brass that I shot over the weekend and grabbed 5 cases that hadn't been squished/stepped on, and those measurements came to:
1.078"
1.077"
1.077"
1.078"
1.077"

I was going to resize them back to 1.074"

These were fired from a home built upper - Aero receiver, BA barrel, and a BCG from DSG Arms (Unsure on brand)

My measurements all were consistent, so I'm think I'm OK but the very small shoulder of the 300 gave me pause to verify with some of you.

Thanks for any input.

Two 300 Blackout uppers from the same manufacturer, not yours, and using the Hornady Headspace Gauge if I size to 1.077" the cases will just barely chamber and tight. Fired cases are about the same 1.077" so at this length I'm not pushing shoulders back little or none. Ruger Ranch rifle bolt action is tight to chamber at 1.073". So I size to 1.073" plus nothing minus one or two for the AR chambers and bolt action.  No problems sizing shoulders 4-5 thousandths less. Couple factory roumds I gauged were 1.068"- 1.069". Your S&B factory cases are pretty short. Seems 300 BLK chambers vary a bit from one rifle to the next?