On a rifle, or pistol, chambered for a rimmed, or semi-rimmed, cartridge (the 30-40 is rimmed) the head space is from the bolt face to the receiver face.
On a Belted case the headspace is from the boltface to the bottom of the belt recess.
Auto pistols headspace from the boltface to the end of the chamber.
Rimless bottle neck cartridges headspace to a datum point on the shoulder.
The reason for removing bolt parts is to eliminate as many sources of drag as possable, as checking headspace is a "feel" thing.
Rimmed and belted cases can be checked by using a F/L sized case and some tape (or a ring of thin solder) between the casehead and boltface. Verify what the dim is supposed to be, measure the rim (belt), add tape until you feel resistance to closing, and measure again (with the solder just crush it with the bolt and add the crushed dim to the rim thickness)
Belted cases often have there shoulders set way back when sized, which leads to their noted short case life (just touching the shoulder when sizing will extend their case life greatly)