Here is a tib bit from a rifle shooter magazine article by Patrick Sweeney.
"In simple terms, headspace is the dimension of the chamber of your rifle, the gap between the face of the bolt and the stopping surface for the cartridge. To be precise, it is the distance between the face of the bolt and the datum line, which is a circle of stated diameter, along the slope of the shoulder of the cartridge.
On a rimless cartridge such as the .308, the stopping surface is the shoulder. On a rimmed cartridge like the .30-30 or .303 British, the headspace is the gap for the rim. On a belted magnum, headspace is the gap for the belt. Since not every factory rifle or cartridge can be made to exact dimensions, with perhaps the best custom rifles
being one exception, headspace is deemed to be correct if it is within a certain range."
Here is a link to the entire article. Measuring Headspace
If this is a factory rifle you could have the lugs lapped and the barrel cleaned and bore scoped for any unforseen problems.