I'm specifically referring to a Micrometer. Where the screw can develop wear and begin to develop back lash so they are not fixed and will shift.

Every precision measuring tool made should be shipped with a standard.

So you just trust that what the caliper or micrometer says is true with no means to verify it to a known standard?

Ebay has[u][certified/u] gage blocks 36 or 81 piece sets for around $50 and $75 give or take respectively. Being so inexpensive I see no reason not to have a set.

I understand the level of accuracy needed in reloading that's not the issue. Assuming your measuring tool is fixed and rigid and never needs calibration is a huge mistake. And zeroing is not calibration. two very different parts of a verification.