Space between chamber and breach block

[ithacaartist]

Originally Posted by willyboy

This is basically a foreign language to me. Could someone dumb this down for me?

I'll try. Rich is saying there are two measurements to take in this area. Headspace is the nominal dimension between the back end of the cartridge when it's seated properly in the chamber, and the face of the breech block that corresponds to it, the milled out area in its nose, thru which the firing pin protrudes.
Case exposure is how much of the brass case is visible?/unsupported by chamber structure. If you had an abnormally shallow chamber, the back end of the shell would hang out. You could still have a headspace within acceptable parameters if the overall length of the toggle train were short enough, by the amount the rim protrudes, but situations of excessive case exposure can be dangerous.
The pros have standardized data and measurements available to check against when making these measurements.