.40 S&W taper crimp dimensions

Here's a pretty good basic procedure.

Start with an empty case. Seat the bullet until it gets to the OAL you are looking for. Once you have the correct OAL, lock the seating die in place and advance to the crimp die. Screw the crimp die down until it makes contact with the case mouth. Then screw it down a 1/16 of a turn, crimp and measure the case at the mouth. If more than .422 then screw the die in a bit further, crimp and remeasure. Keep repeating until you to get between .418-.422", at that point lock it down.

I take my dial micrometer and check dimensions from base of bullet in the case up to the neck. It should decrease in size near the case mouth and that tells me it is crimping. I then take my barrel out and see if the fit in the chamber OK.

I use .420-.422. No problems so far.

Using you barrel as a case guage will help you determine the amount of crimp needed .... for that particular pistol ....

The theoretical ideal is the diameter of the bullet plus two times the thickness of the case wall. In this case that's .40 + (2*.010). With straight wall pistol cartridges you're not really crimping so much as just removing the bell you put in it to seat the bullet.