[mrerick]

The case separation is happening just above the thicker web near the base of the cartridge. It's an adjustment problem with the die used to draw the cartridge case from a bronze pellet. Over-drawing the cartridge at this point creates a very thin area that can't tolerate the stretching caused by firing.

Your gun's chamber may also be worn too large, making the stretching more extreme. There are techniques for casting the chamber and measuring it for specification.

Base separation is always difficult to clean up after. At least the gun won't generally go back into battery after the failure.

On your first picture of the failing recovered case front, you can also see how far off the position of the cartridge shoulder is. If I remember correctly, this case headspaces on the shoulder of the cartridge. Obviously there is something very wrong here from the shape of the fired cartridge. It's almost as if the gun was out of battery and in the process of extracting while the powder was still burning and expanding, forcing part of the shoulder against the rear chamber wall. This could be caused by defective or contaminated powder burning erratically.