Extractor
A spare part for your shooters bag for sure. I recommend replacing them every 8,000 rounds. The main part I inspect for wear on a regular basis. Irregular and only with bad brass, but case separation can damage the extractor. So far, only happened to me with 40 S&W and very fast burning powders.
It's a good idea to replace the extractor pin and spring every now and then.
If you take the extractor out to clean it on a regular basis, the extractor pin gets worn and eventually will get loose.
If you're having problems with cases getting stuck in the chamber you can try the Wolff extra power extractor spring or modify the extractor.
See article on how to "Tune for Rimless".
This concept can be applied to all caliber and helps put more tension on the extractor and helps pull empty cases out of the chamber.