[renaissance7697]

When You crimp:
The die squeezes down the brass AND the lead in the bullet
( ALL [almost] bullets are lead in the core )
When you pull the completed bullet out of the crimp die
The BRASS tends to "Spring back" a little
The Crushed lead core DOES NOT!
If you crimp too hard, the bullet winds up "loose" in the case mouth.
It tends to move under recoil.

Unless you are ROLL crimping into a cannonelure (sp?) [ = the little 'groove " in the bullet ]
You should only crimp enough to remove the "flare" you introduced when preparing the case to recieve the bullet.
Returning the case to it's original "sized" diameter.

The Lee FCD ( Factory Crimp Die ) does this very nicely.
Of course you have to seat and crimp separately to use the FCD.
Seating and crimping separately removes a LOT of potential problems.