One thing you will realize is that all 9mm brass is not the same there is a lot of variation in case length and thickness all which will have some affect on your taper crimp setting.
In reloading straight wall cases that headspace on the case mouth we use a “taper crimp” die. However this die doesn’t really “crimp” anything. It’s sole purpose in life is to remove the flair on the mouth of the casing so the round feeds and headspaces correctly.
A properly adjusted taper crimp die may or may not increase bullet tension by as much as 10%. Screw that die down further and the casing starts to buckle releasing it’s grip of the bullet.
Bullet tension is determined by:
1. The resizing die.
2. The expander plug. (Bell mouth die)
3. Case wall thickness.
4. Bullet diameter.
The best way to properly set up your taper crimp die is first resize a casing and measure the mouth diameter with a set of calipers. Now take that empty casing and run it through the crimp die and measure again. The two measurements should be the same, or up to .002 smaller,less is more when it comes to taper crimping.