The Lee FCD dies have a carbide ring for post crimping sizing. It is believed to be for insurance that all rounds will chamber as the ring sizes the round to "standard". I tried an FCD and found it ruined my cast bullet loads. I knocked out the sizing ring and the rounds were no longer undersize, but I wasn't impressed with the crimp. I have been reloading semi-auto rounds since '88-'89 and have never needed to post crimp resize any ammo (380 Auto, 9mm, 45 ACP). I separate seating and crimping because early on in my reloading I had more control over both steps and today I have some preferred crimping dies other than stock crimps. I like Redding Profile Crimp ans Lee Collet Crimp, and a couple "stock" taper crimp dies, RCBS and Hornady, so separating steps is a good "habit" for me...