Hardens, weakens,etches, I don't really know. This was the only explaination I could come up with that would cause my case heads to separate the way they did. My loads aren't that heavy, and they were once fired. I know this because they were still in the original Federal boxes that I bought them in. I always use the plastic boxes. I found these in a box that was shelved about 10yrs before I started reloading for the 35. I always saved them just in case. KN