Back in the ol' days, when we didn't have the luxury of crimping as a separate operation, we'd adjust our combination seating/crimping die to remove the flare, hopefully without cramming the case mouth into the bullet too much. Then Mike Dillon came along and built machines that seated and crimped in separate stations! Wow - crazy stuff. And then he even started making dies that crimped auto-pistol cartridges correctly, that is to say didn't actually "crimp" at all, but simply removed the flare from the case, laying the (flared) case mouth perfectly flat against the bullet.
Unfortunately, somewhere during the taper crimp die's evolution (from the roll crimp die), no one bothered to rename it so as to not mislead folks, for generations.
Roll crimping should do just what the name implies - the case mouth is slightly (or sometimes drastically, in the case of a heavy recoiling revolver) rolled into the bullet's cannelure. Which helps prevent the bullet (in the unfired rounds) from moving forward in the case in a revolver. Since revolvers typically headspace on the rim, as long as the roll crimp doesn't deform the bullet's bearing surface, no harm is done with even a heavy roll crimp.
Since autoloaders headspace on the case mouth, however, the case mouth should never be crimped so that the case mouth penetrates into the bearing surface of the bullet. So in the case of an autoloading pistol, the "taper crimp die" should never actually crimp the case mouth into the bullet in any way whatsoever. The roll of the taper crimp die is to remove the flare previously applied to the case (in order to seat the bullet without shaving copper/lead) - returning the case mouth so that is snug down on the bullet, but not crimping the case mouth into the bullet at all.
After some experience one can learn to "see" (without tools) if this has been done correctly. (Either too much or too little.) But until then a good set of calipers will guide you as you adjust the "Flare Removing" die. Using the thin part of the calipers (near the tip), measure the loaded, flare-removed round's case mouth at the very end of the case. The result should be the sum of the bullet's diameter plus 2 times the case mouth's thickness. (Usually .010" for most cases. So in the 40 S&W, the Flare Removing measurement should be approx. .420". Or at the tightest, .419". And be sure to "roll the case mouth around," so you don't just measure it in one spot.
Actually (taper) crimping the case mouth into the bullet almost always results in a loss of accuracy. Not removing the flare enough results in malfunctions. Some gunsmith's will recommend "taper crimping" more than what was previously recommended, and that is easy to understand as to why.
If you remove the flare precisely as outlined as above and your pistol (still) has any sort of feeding malfunctions, I can say for sure that crimping the case mouth more is not the solution. Once I learned how to properly set this dimension, I never experienced any malfunctions due to not enough "taper crimp" in over 20+ years of shooting 20-40,000 rounds/year.
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