So I ordered a new upper on black Friday for cheap. Everything worked great except it took 27 clicks off center to get the rear sight on. I sent the sellers a question asking if there was something I could do to bring it back closer to center. I still had some clicks left before hitting the side of the carry handle/sight but it looks sorta funny. Thing shot like a laser and I could not see anything off center. . At any rate they responded with a pre paid label to send it back. My question is how would a mfg. fix a canted FSB? Since it was drilled and pinned on I was just curious how that was accomplished. Do they just throw the barrel away and mount another?

The barrel was probably overtorqued during assembly.   They will just back it off a bit and retighten to spec.

All true. I really intended to just shoot it. It was distracting to have the aperture so close to the left side and have everyone who picked it up say "why is the sight so far over". However when they responded to my question with -we are emailing you a label and will fix it -I figured OK.  May or may not have been a good call.

I have heard of folks grinding the indexing slot in the upper wider so that the barrel could be clocked.

Actually...  When it's close, and the barrel nut isn't too tight, this can work.  i got the idea from a TV show in which they toured the FN plant and I saw this in the background.  They just whacked the FSB to move it a bit.  They also showed the FN personel using a slip fit rod to test barrel straightness and bending the barrel till the rod slipped though.