Thanks Brian, I was hoping you didn't have that many personal head seperation problems.
Anyway, Brian picked out some good examples.
In passing, I like to section brass and bullets, and cases such as were shown on Brian's post really give a good picture of what is taking place, when sectioned.
Just for James' info as he seems to be possibly a newer reloader -------------
What happen possibly over time with a good number of firings, or possibly very quickly if the dies are not properly set is, the brass is pushed forward by the firing pin until it is stopped by the shoulder during firing, the pressure of the fired powder expands the brass to where it grips the chamber wall and then the unsupported head of the case is slammed back to where it is stopped by the bolt face.
If the sizing die is not set properly to match the chamber, (some manufactures instructions are bad this way) the brass then may be over sized during sizing and the above process is repeated at the next firing.
This streaches the brass just ahead of the case head each time it is fired until the head begins to seperate.
Also along with this, during the sizing, the brass moves forward more rapidly then normal, causing increased need for neck trimming.
Crusty Deary Ol'Coot