Adding to what Randy said… I read on here one time that factory ammo uses some sort of a coating on either the bullets of inside of the brass to increase the friction between the two, thereby reducing setback. I had previously figured this was the case, based on some .357 Sig ammo I had seen.
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Close the case mouth just to the point that it is no longer the largest diameter. You can over-crimp to the point that it will weaken the joint and increase setback (due to poisson effect).
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Measuring the diameter of the case mouth is tricky, and dependent upon your brass thickness. As shown in the image below, I like to hold the round in the jaws of a caliper to confirm the mouth is just ever so slightly crimped. Looking at the bottom jaw of the calipers, and you can see light passing between the jaw and the case mouth. Another thing that can be done here is to pivot the round while in the jaws of the caliper (pivot in an axis perpendicular to the case’s cylindrical axis); the pivot point will be where the largest diameter is. If the case mouth pivots outside of the jaws, then the largest diameter is behind the case mouth.
Attachment 317566