It does not take much pressure. Just light yet firm contact. Do not crank it down or force it.
Most calipers have flats and blade sections. I measure most things with the flats, especially COL and case length. If you use the lade section you may not get exactly on the hihest point each time.
Even though case mouths may look even, there can be small variations not noticeable to the eye. The flats will more likely hit on the highest point each time whereas the blades can end up in slight depression.
If you are measuring diameter then have the diameter in pararell with the lades rather than perpendicular so the highest point will defenitely be under the caliper and you are not guessing when you have the greatest point.
Also, be sure that it is zeroed before each use and lock the zero ring.
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