Probably just "peening" from the bottom of the casing during recoil. The casings in each chamber bounce around during firing and when the gun is being carried. More of a "stain" or burnish really......just evidence of contact between the casing and the breechface.
Take a look at it with a magnifier, it should be superficial only.
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Ret. LE, FA Instr, S&W Armorer
Last edited by armorer951; 09-02-2015 at 11:51 AM.
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