I had never seen this before, and thought it might be interesting: So there I am, at the range, firing a carefully worked up experiment over the chrony. .223 Remington, <0.002 headspace, S&B SRP, 69gr BTHP, H4895, 1.2 grains below this rifle's pressure sign threshold. . . and this pops out of the chamber: What in the world? Chrony says velocity is right on target with the rest of the batch, all the other primers are healthy, and the primer appears to be collapsed IN, not out. Two more occur in the rest of the batch, and now I'm really wondering. A few minutes at home with a decapping die and loupe. . . and I solved it. The primers look like this: Notice that the bulge around the rim shouldn't be possible, because the pocket should be in the way; also note that the primer skirt appears shorter than it should be, exposing the anvil feet. And here's why: This batch of brass was swaged (on a Dillon, not by me), and apparently at least three of the batch were slightly overswaged, pushing what should be the entry radius down into the pocket. The firing pressure expanded the primer rim out into that void and pulled the primer skirt up in the process. Here's a property swaged example to compare: There were no apparent ill effects, but this is a failure mode I have never seen before.