Lee factory crimp dies are great. I buy one for all the handgun calibers i load. I use the 3 carbide die sets from lee, I add the forth being the factory crimp die.
Autos use a taper crimp, Revolvers use a roll crimp. The lee dies give you these crimps as they are caliber specific. No guess work, if you have a progresive turret type price, you add this to your tool head. This die will be the one after the seating die. You seat then you go to this next die and perform your crimp.
You can adjust the die for a light crimp or heavier crimp, by turning thr knob as you do with your seating die.What this die also does is make all your ammo universal as to function in all like caliber pistols. Hence the word "factory" in its normanclature. Basically it will make your auto and revolver rounds more reliable. When you start using these dies, you will notice that some rounds will feel like they are being crimped more than others.
Although you have the crimp knob setting at the top of the die at the same setting, some rounds will get more attention than others. My theory is that some of the brass might be that little bit longer than the rest. Or the bullet projectile is that little bit more or less in circumfrence.
All i know is that your ammo will be better in feeding and unerversal. Have you every given a reload to another shooter with a different make gun in the same caliber, and for some reason it does not feed right in his or visa versa?
That is the Factory aspect of the design at work. I do not have any go no go type guages for any of my reloads. I stay within the perometer of the OAL given in the load data, and respect to thier oal regarding different powders, as not to raise pressures. Not exceding the Max oal.
I have been reloading pistol ammo for 25 yrs, and use Lee dies exclusively. 4 dies to complete the process. I have been reloading rifle for a couple of years now and find the big difference is the rifle cases, especially the the AK and Ar shouldered cases stretch after following. This is mostly due to thier shoulder type cases and more powder resulting in higher pressures. So you need to trim, you rarely if every need to trim pistol brass.
Getting back on the crimping, i do not use a alot of crimp when i adjust the crimp die, they say the brass will last a little longer. I really do not know how many times you can reload a cartridge, depends on how hot you are reloading