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What's up with chamber?

915 1
Hi from beautiful, warm (finally) Montana,

A question for the board, if I may. My rifle, new, had a headspace of 1.632". I loaded rounds with a cartridge headspace of 1.624".
Why should they show signs very much like peening on the shoulder and rims chewed up? The problem showed itself after firing. In trying to determine the cause I would load and extract without firing and the cartridges looked the same. I am in the middle of barrel break in, per M/Sgt Owens, and chamber was clean and dry. Any help in this matter would be greatly appreciated.