I was reading this thread the other day just out of interest in things MP5 as I have a real one and four nice clones all outfitted with the best in German technology.
I've had one gun that has had weird and erratic feeding/double stacking/cases dribbling/cases getting stuck. Its one of IGF Jayson's guns from way before the current implosion and has always run fine... I got fixated on ammunition (round nose vs. truncated), running suppressed or not, and straight vs. curved German HK mags. KCI and POF's have always worked fine for me too.
So gun goes into single shot, jam, clear single shot mode and I'm just lost until I take the carrier/bolt assembly out and the extractor fall on the bench with one half of the extractor spring. Replaced it with a new one and that's all it was.
Sometimes it is just the simple, already well known and documented fix. I'm ordering 10 springs today and replacing all of my old ones and will have a few as spares in each gun bag.
A lot of you alluded to this but it just didn't connect until I actually saw the pieces on the bench... I would have felt pretty stupid if I had the gun in my hands when the living dead attacked...