I think its called case head expansion. Loaded (Federal) factory ammo I have measures .459 just below the rim whereas the Rem and Winchester measure .451. The brass after firing is .460-.461.. the Rem and Winchester both measure .457-.458. This is measuredat the biggest part of the case just below the rim and just ahead of where the bolt covers.


This gun was bought in 1996 and has probably had 1500-2000 rounds fired thru it and the brass has always measured the same on the expansion part. Thats why I was wondering if the neck sizing would be a good step to make the brass last longer.