The pressure which is measured by a sensor within the Gun.

As the gun contains multiple components, care should be taken about where exactly in the gun pressure is measured e.g. at the source.

Custom modifications of a microscope can lead to a more accurate pressure measurement. If this is the case it will typically be detailed in the methods section of a paper.

Ideally gun pressure should be directly measured inside the gun; however this is often not the case and pressure is measured by a sensor located outside (but close to) the gun chamber due to manufacturer design decisions.