The 7mmRM is said to headspace on the belt. That is for sure on the first shot, because the shoulder on a new case will not likely reach the shoulder in the chamber, before the belt on the case can reach the belt in the chamber.
That is not to say that the case belt and the chamber belt are close, they are not. SAAMI specs are sloppy
.220 - .008" for cartridge belt
.220 + .007" for chamber belt clearance
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Belt specified to have between .000" and .015" clearance
2.2700 - .0070" for cartridge shoulder clearance
2.2791 + .0100" for chamber shoulder clearance
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Shoulder specified to have between .0091" and .0261" clearance
The brass I measure of many samples of many brands looks like .210" to .215".
So when I cut threads and the cut chamber on a 7mmRemMag rifle, I headspace the rifle at .215".
But the factory rifles are all more than .220"
I have a Ruger #1 7mmRemMag.
So what to do with a factory rifle?
The first time brass is fired, the belt will stop the forward movement of the cartridge when the firing pin pushes.
Partial neck size, neck size, or FL size and push the shoulder back .001".
Then on the next firing, the shoulder will be what stops the cartridge forward movement in reaction to the firing pin.
What does it all mean?
If you touch the die to the shell holder, after a few shots, the case may break in half.
You have to screw the sizer die a little higher in the press.