Discussions of headspace tend to be confusing because the speakers often go too soon to how it's measured without a clear explaination of what it propery is.
Simply stated, "headspace" is where the cartridge sits in a chamber. Eagletts drawings clearly illustrate that. If there is too little headspace the action cannot be closed on a standard cartridge case. If the headspace is too great, the case will move too far forward in the chamber and a lot of non-funny things become possible with firing. Headspace is measured from the bolt face to the part of the chamber that prevents further forward movement of the case.
Almost everyone knows rimmed cases headspace on the rim itself so a case body that fits somewhat loosely in the chamber is not, of itself, a hazard. Ditto with belted cases, belts are actually sort of funny looking rims. Rimless, or rebated, cases must stop their forward motion on either a shoulder (bottle necked cases) or on the mouth of the case (straight walled cases).
There are different ways of measuring it but headspace gages are not very good, they don't actually tell us WHAT the headspace is, only if it lies between a specified minimum and maximum length. I want to know if my chambers are on the minimum or maximum side of that specified range so I can set my FL dies accordingly!
Perhaps those tools that measure fired case lenght to the shoulder, on bottle neck cases anyway, are best for measuring true headspace, it will be perhaps a thousanth or two more than the measured length because of brass spring back after firing. I believe the "Precision Case Mik" tools from RCBS are perhaps the easiest way to measure the location of a fired - or resized - rifle case's shoulder. The Sinclair and StoneyPoint/Hornady case gages are close seconds.
Sinclair sells chamber length gages, small cylindrical brass pieces to be seated in place of a bullet, which can be used to measure the OAL of a chamber. Rifle chambers are typically 20-30 thousanths longer than minimum so that is good info for case length trimming but it only relates to headspace, as such, for straight sided pistol cases.
Last edited by boomtube; 07-11-2008 at 07:53 PM.