Failure to Extract (Often Incorrectly called a Double Feed)

Here is a prefect example of what many people call "Double Feed"

This is a failure to extract, if you look you can see the fired primer on the casing, this has failed to extract and the next round is trying to feed under neat.

Normally we would remove and inspect the the extractor. Personally I would clean the extractor channel and replace the extractor and spring.

In 9mm we have never found it necessary to use an extra power spring.