Why is it not advisable to headspace a cartridge with the extractor in place?

Headspacing a cartridge with the extractor in place is problematic primarily because the extractor can alter the measurement by not allowing the cartridge to seat properly against the breech face. When the extractor is in place, it applies pressure to the rim of the cartridge, which can create a gap between the cartridge base and the breech face. This results in an inaccurate headspace reading, as the measurement will reflect the space created by the extractor instead of the actual alignment between the cartridge base and the breech face.

Achieving accurate headspace is crucial for reliable firearm operation, as it ensures that the firing pin strikes the primer properly and that the cartridge is securely held in place during firing. Therefore, ensuring that the extractor is removed during headspace measurement is essential for obtaining an accurate assessment. Such precision helps prevent potential issues during firearm operation, such as increased risks of misfires or failures to eject.