Originally Posted by
exkc135driver
The "heat mark" is caused in the manufacturing process when the brass is annealed to soften it somewhat in the neck and shoulder area. Thus, a better gas seal is obtained and the case neck is less likely to split.
Every time the case is reloaded and fired the brass, especially in the neck and shoulder area, work-hardens, so after it has been reloaded a number of times it should be re-annealed. But that does not mean that it needs to be re-annealed every, or every other, or even every third reloading.
Some people who reload tumble their brass just enough to clean it up, in which case the annealing will still be visible. Others, including some commercial reloaders, like their brass to look all bright and shiny, so they tumble it more, which causes the brass to look like the brass in the photo above.
I'm not saying that this is what happened or that the brass was reloaded. I'm just explaining the process.
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