Originally Posted by
Coal Dragger
I would get the chamber headspace in that barrel verified and get a solid number on it so you know what you’re dealing with.
Additionally if you don’t have one, you need to buy a case gauge in .300 Blk to set your resizing die so that you know you’re not bumping the shoulder back too much on your brass. Headspace issues can also be out of spec ammo, so you have to verify both. Good news is that case gauges are not expensive nor difficult to use, and will more often than not help correct all sorts of nasty function issues in semiautomatic rifles caused by improper case dimensions of loaded ammunition.
Anecdotally, when I started loading .223/5.56 the resizing die I was using was bumping the shoulder back too much, and I was getting backed out primers, brass flow into the ejector, and extractor swipes on the brass. Just like your fired cases. Setting the die to turn out properly resized brass verified with a case gauge solved those issues.
Additionally one other thing that can cause primers to back out is priming brass that had a crimped primer without removing the crimp. The primer cup just swages down to the crimp diameter and doesn’t get a good grip on the primer pocket and they back out. So swage or cut the crimps out of the primer pockets if the brass has crimped primer pockets.
As others said sort your damn brass, and work up from starting loads. Be safe.
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