Crimps don't get or need to be measured.
I disagree.
Since getting my Redding micrometer taper crimp die I record its setting when loading my 9mm ammunition. I set the crimp so as to remove all mouth flair AND not deform the bullet diameters (beyond the deformation that resulted systemically). As most of us do, I save and record in my log other significant measurements such as over all length and the brand of cases. But I also have started measuring and recording case wall thickness. This saves me time when I switch from bullet to bullet -- I just look up the taper crimp setting for that bullet with a particular brand of case.
Here is how it all comes into play.
I have three 9mm pistols. They have barrels measure .3548", .3552 and .3568" in groove diameter. I have lead bullets and poly coated lead bullets that range from .3546" to .3573" (these are averages developed from measuring five random bullets, in a box of bullets, at three 120 degree locations on those five bullets). I also have .356" Hornady XTP and .355" Montana Gold JHP bullets.
The cases I use are Winchester, Starline, R-P, Speer/Blazer, S&B and PMC. They have varying case wall thicknesses. They vary from .0085" to .0105".
One of the pistols has a slightly short free bore but average size chamber; it has the .3568" groove diameter. One has a long free bore and slightly large chamber; it has a .3552" groove diameter. The third has an average free bore and average size chamber; it has the .3548" groove diameter. My assessments of the free bore sizes came from trying different lengths of .356" XTP bullets in Speer cases doing a plunk and rotate test. They are purely judgement calls.
I have learned the hard way that the taper crimp adjustment has to match both the bullet diameter and the case wall thickness. Why? Using the same size taper crimp that I used on a hard smaller diameter bullet in a case with a thin case wall on a soft fatter bullet in a case with a thicker case wall can deform them to the point that they lose some accuracy. And we all know not removing enough mouth case flair can affect feeding reliability.
By using a micrometer taper crimp die I can control how much the mouth is turned in and eliminate any squeezing of the bullet at the case mouth. This step is recorded and is repeatable. I can go back to a load and quickly reset the taper crimp for any combination of cases and bullet diameter that I previously loaded.
I still have to match the barrel groove diameter with the bullet diameters AND take into account the hardness of the bullets (as a soft larger diameter bullet in a thicker case will be squeezed down in diameter more than a hard smaller diameter bullet in a thinner case). But that is all part of the art of reloading.
Hopefully they explain the benefits of a 'micrometer' seating die for 9mm
'Micrometer' seating is another topic. But here's a hint as to why you might want to use one: differing free bore lengths.
