Bigcountry:

When one full sizes, thewhole case is being worked. When oneneck sizes only the neck is being worked. When the case is fire formed, the brass flows towards the neck causing the case tobecome longer, and expand to the size of the chamber.Full sizing causes the case to go back to SAAMI specifications. Continuously full sizing makes the brass expand and contract over and over again.If onebends a piece of wire back and forth over and over again it breaks, the same happens with brass.

Between the head and the begining of the case body is where the brass becomes very thin causing case head seperation. Between the case head and the begining of the body is the weakest part of the case because the case transforms from a thicker portion to a thinner portion.Due tothe brass being fire formed and full sized, the area between the case head and the begining of the case body thins, andweakensbecauseit'sbeing over worked just like the wire and finaly breaks. The area between the case head and the begining of the case body is more vulnerable todamagethan the neckbecause ofdesign, and metal fatiguecaused byfull sizing.

I have thrown away more brass away due to head case seperation than split necks.After every third firing I check for headcase seperation with my RCBS Case Master.Good luck.