From Varmint Al's website:
http://www.varmintal.com/arelo.htm#Polish_Chamber
'POLISHING MY CHAMBERS.... When I got my 223 Ackley Improved Virgin Valley,no longer in business) barrel, it had a very rough chamber. The fired brass had marks I could see where it had been gripped by the chamber and then slid backwards a tiny bit until stopped by the bolt face. It looked almost sandblasted. There were protruding primers on some of the fired rounds. When the firing pin hits the primer, it pushes the primer and the new case forward in the chamber. When ignition occurs, the pressure expands the walls of the brass. The rough chamber, with its high friction, grabs the case at the forward position and the gas pressure first pushes the primer back against the bolt face. As the pressure builds, the brass case is stretched until the case head is pushed back against the bolt or the case supports the total load. If the case head is pushed against the bolt face, it would produce a flat primer, but flush. If the load is light, the case does not stretch so much and leaves a gap between the case head and the bolt face, but the primer is still against the bolt face. This produces the protruding primers.'
Another bit of info:
http://www.varmintal.com/arelo.htm#CaseHead
http://www.varmintal.com/arelo.htm#Polish_Chamber
'POLISHING MY CHAMBERS.... When I got my 223 Ackley Improved Virgin Valley,no longer in business) barrel, it had a very rough chamber. The fired brass had marks I could see where it had been gripped by the chamber and then slid backwards a tiny bit until stopped by the bolt face. It looked almost sandblasted. There were protruding primers on some of the fired rounds. When the firing pin hits the primer, it pushes the primer and the new case forward in the chamber. When ignition occurs, the pressure expands the walls of the brass. The rough chamber, with its high friction, grabs the case at the forward position and the gas pressure first pushes the primer back against the bolt face. As the pressure builds, the brass case is stretched until the case head is pushed back against the bolt or the case supports the total load. If the case head is pushed against the bolt face, it would produce a flat primer, but flush. If the load is light, the case does not stretch so much and leaves a gap between the case head and the bolt face, but the primer is still against the bolt face. This produces the protruding primers.'
Another bit of info:
http://www.varmintal.com/arelo.htm#CaseHead
