When they are brand new I clean it before taking it out for the first time. Shoot the crap out of it. Bring it home and clean the crap out of it. One thing I don't do is get the barrel real hot real fast. Let it gradually get hot by shooting slow the first time out. But thats just me
I just clean a new barrel and zero it in. No special break in.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek