Creepy 45ACP cases found at the range

[Questor]

I found a bunch of aluminum non-reloadable 45ACP cases at the range yesterday. Maybe they're Blazer cases, the logo on the casehead looks kind of like the Briggs-Stratton logo, but I couldn't read it.  The primers are brass colored.

Every one of them, and there were about 50, had been fired and the dent in the primer had been blown back out so that the primer was flattened out again almost as if unfired, except for marks showing that the firing pin had hit. A bunch of Winchester brass was among these and I guess that the guy who fired them had a box of the aluminum cased cartridges and a box of the Winchesters and fired them both from the same gun.  

What explains the dent in the primer cup popping back out like that?  Obviously these were not handloads of any kind.

[Reed1911]

Higher pressure than the primer could handle. That is the only way I know of how to flatten the dent out. NOW that does not mean that the cartridge was over pressured, though it is a very good indication of it. There are times, especially with fine ball powder that enough powder leaks into the primer cavity and will cause them to flatten. Chances are it is the former, but the later is a known issue.

[Mark whiz]

Blazer ammo is usually loaded pretty warm (at least most of the stuff I've shot has been) so I would figure that the dent being pushed back out is either a function of the high pressure of the round or maybe the wierd flash hole a lot of this brass has - more of a slot than a hole.