Re: 6mmBR loads
The most common amount of brass spring back is .001".
Thin brass springs more.
Large diameter springs more.
Work hardened brass springs more.
I like to measure chamber necks with pin gauges.
I originally got pin gauges for lathe and mill work, but now have a set in the reloading room.
If you knew the chamber and there was .001" clearance with the measured loaded ammo necks, that would be enough clearance.
I have a 6mmBR reamer with a .272" neck.
I have a Forster 6mmBR sizer die with a .265" neck [measured with pin gauges].
I have enough clearance with RP or Lapua brass, no turning required.