If the case had a radial split, I'd say the rifle did NOT fire out of battery and what you have is a simple case of bad ammo. Just brittle brass that cracked under pressure. It doesn't take much of a puff of gas to blow the guts out of a magazine, and the primer probably just dropped out afterwards because the pocket spread wider.
If it really fired out of battery, which could only be due to a broken firing pin, a bur or bump on the bolt face, or a high primer, then I would think you'd see noticeable damage to the bolt or receiver from the violence of the blowback. Also, the case head would almost certainly have blown off completely, probably leaving the front part in the chamber. Even DELAYED blowback firearms need waxed ammo or fluted chambers to prevent head separations, when firing high-intensity cartridges.