Found this in my brass bucket, what caused it?

[Kperkins]

I would have thought the case would have ruptured more as well. I'm just glad nothing catastrophic happened. That could have been bad

 

[russc2542]

I would have thought the case would have ruptured more as well. I'm just glad nothing catastrophic happened. That could have been bad

One of those things that just depends on a hundred factors. once things start misaligning all the rules of thumb and baseline graphs go out the window. by not having the bolt locked and case fully in the chamber, the peak pressure's reduced: chamber volume's greater by however far from fully seated against the headspace and the case recoils immediately rather than the locked bolt holding it in place until the pressure dissipates and the bullet's halfway down or out of the barrel, increasing it further. the bullet isn't in the throat sealing the bore so some gas might escape around it before it does so. that's just what I can come up with on a whim.

 

[NKBJ]

And the front of the case collapses and some gas bypasses it coming out of the rear of the chamber...
If it isn't fully in battery then most anything can happen.