Bad brass. Not necessarily their fault, because it most likely looked perfect when it went into the box.
I had a bad lot of Winchester 7.62x54R. Reloaded it and shot a few, tossed the mosin in the closet for a rainy day. Pulled it out 10+ years later and had the same thing. Virgin brass I reloaded myself, 1/2 of the case necks had split. I shot the survivors and all that brass went into the recycle bucket.
You could give them a try. Not like the case can really go anywhere once it's in the chamber, but you may get some gas venting. I was tempted to try it in the mosin until my buddy informed me they were $400+, not the $75 I had paid for it. Not saying it's a good idea, but it may provide us some entertainment if you live to tell the tale