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?