Ensci will give a more exact scientific answer but we suspected that overtightened breeching up was the problem. Any 'bulge' was caused by the inability of the material to resume its natural state. The MAG (the Maintenance Advisory Groups) who worked with the RSAF took some chamber castings in alloy but even then they were tight to remove! I had/still have somewhere one of the bulged chamber sections that I machined down to make a simple calibration cup for the various 7.62mm CHS gauges