Myth or Fact?

I can only offer a guess from one experience trying to remove the primer crimps on very old 7.62 LC brass (1968), swaged in 2016.

I had a slam fire (AR 10 clone) from a high primer in a batch I loaded on my progressive after swaging. A double tap is unnerving, so I investigated.... In that batch I found 3 more high primers, not yet fired. So I took those rounds apart.

I found a tiny ring of brass at the bottom of the flash hole that just spun in the bottom of the hole, when I had attempted to "uniform" the pocket hole depth during case prep previous to reloading them.......so then the uniformer didn't cut only spun in the holes, and the primers were high. Un-uniformed pocket plus the brass sliver left in there too will obviously do that. I found the same thing on the other 2, and also the shot case that had caused the double tap, found when I de-primed it.

So obviously, when the brass is too hard, it can shear rather than reform back to the proper un-crimped diameter, leaving the sheared ring behind....in the bottom of the primer pocket.

The 7.62 LC brass was once-shot picked up at a National Guard range, but is was stored and very old. So work hardening was not the case....just age.