Ive seen some threads say crimping is not necessary on pistols
Sorta , sometimes. The reloaders that 'don't' need to crimp, don't bell the case-mouth for seating the bullet. For me, some bullets are okay with this and some aren't. My personal choice is to use the smallest possible bell to 'comfortably align the bullet for seating. JUST my choice.
Now that I've belled the case mouth, I have to 'taper crimp' to JUST close the belling back snug against the bullet.
Two methods come with LEE 4-die sets:
1. Use the seating-taper crimp die to BOTH close the bell and seat the bullet. The stem height drives the seating. The die height drives the taper crimp.
-- Some reloaders find they don't prefer seating and crimping in one pull. They find it more precise to do these as two separate steps.
-- Case length variations affect the taper crimp. Longer cases get more crimp than shorter cases. OAL is not altered, just how deep the bullet is seated is affected by case length. Longer cases seat deeper than shorter cases. The free-air space above the powder is NOT affected by case length.
2. The LEE FCD die can be used for a separate taper crimp function. Again the die height determines how deep the case goes into the die, and adjusting the center stem sets the amount of taper crimp.
-- The FCD ALSO has a carbide ring that 'sizes' the case (with the bullet in it) to meet SAAMI max cartridge diameter standards.
Here is where the trouble/controversy starts. The die needs to be carefully adjusted for both die height and taper crimp desired. Mis-adjusted, it can squeeze the crap out of the crimped area where the bullet is seated and even damage or resize the bullet. Longer/Shorter cases also have slightly different results.
-- My 9mm experiences with the FCD have been good. I HAVE NOT loaded LEAD BULLETS. I seat .356" plated Berry's bullets in WIN/WCC moderately thick cases with no marks of any kind imprinted on the bullet.
For either method used, I measure the taper crimp at the mouth of the case. For .355-.356 plated bullets .378 to .379ish" is good in my combo. With .355" jacketed bullets .3765" to .378" taper crimps are good.
An alternate available taper crimp method is noted by Wobbly, remove the FCD and install a taper crimp-only die. Case length variations will slightly affect the amount of crimp.
For other calibers or use with lead bullets, I can't comment.