What happens when headspacing a cartridge with the extractor installed?

When headspacing a cartridge with the extractor installed, the measurement reads off the extractor. This is because the extractor can alter the way the cartridge positions itself in relation to the chamber. When you take the headspace measurement with the extractor in place, the part of the cartridge that makes contact with the bolt face is pushed outward slightly by the extractor. This can lead to an inaccurate measurement since the cartridge may not sit fully flush against the breech face.

In headspacing, the ideal scenario is to ensure that the cartridge is accurately seated in the chamber, which is best done without any components that could influence its position, such as the extractor. Thus, for precise headspace measurements, the extractor's presence complicates the reading.

Removing the extractor before headspacing ensures that the cartridge can be properly measured based solely on its contact with the breech face, giving a true representation of the chamber's dimensions relative to the cartridge.