Ok, about a year ago, I gave my friend my old (about 2,500 rounds fired) DPMS upper complete with original DPMS bolt.  He went out and got a DMPS collapsible stock lower and assembled a nice little 16" A2 style carbine.

Shortly after he got it, he took a carbine class with this same rifle and was experiencing failures to extract about one in ten rounds with Q3131A ammo.  I replaced his extractor spring and checked the gun for lubrication, but the thing still had issues with rounds failing to extract.  He finished the class with my Cav Arms SEBR (#008) with a DPMS upper and had zero issues.

Ok, fast forward to now.  I finally get him to bring the rifle and his mags/ammo to the range so I can trouble shoot his gun.  It feeds, fires, and ejects 60+ rounds of Radway Green without issue.  It does the same with PCA and Remington Yellow box ammo.  However, with the Q3131A, the FTEs return.  I drop in my extractor and his works fine with ALL ammo, even the Q3131A.

My conclusion - Between the 2,500 rounds I put through the upper and the 1k+ more he put through, the extractor was worn a bit and needed to be replaced.  I also told him to get a spring and other miscellaneous parts kit, in case some other stuff needs help.

Did I steer him right?

Do the repair and recheck,what else can you do.

Not to be evasive... Probably. The problem might have been the extractor spring and not the part itself. but if you changed both and the problem went away, I'd say no harm done.