The current offerings recently under went a name change:
ACC to CC
APC to PC
APR to PR
Basically they just dropped the "advanced" from the acronyms. The CC is 16", lightweight profile, chrome lined. The PC is 16", heavier profile, non chrome lined (there was a small early batch that was chrome lined), and the PR is a 20" heavy barrel non chrome lined.
The last generation I'm not as familiar with, there was:
The EC (enhanced carbine) which I believe was a 16" non-chrome lined barrel
ECR (enhanced combat rifle) which was a 20" barrel, not sure if chrome lined or not or how heavy the profile was
Then there was the EMC and ECC (Enhanced Match Carbine and Enhanced Combat Carbine) which were both 16". I forgot which ones were dimpled, chrome lined, etc between them.
I believe the ECR and ECC kind of bridged the two eras as some late examples had the newer bolt style.
I'm sure someone will come along and have the exact answers on the previous generation that I'm foggy on.