[mrerick]

I have seen this type of failure before and can explain what it is probably related to.

When there has been wear along the toggle train - literally in any of the components including the front breech block, front axle pin, middle toggle block, middle axle pin, rear toggle, rear axle pin or frame where the rear axle pin mounts - there can be increased forward to rearward play in the toggle train.

The Luger action will tolerate this forward / rearward play to some extent but as the wear increases over time this can lead to a condition where the cartridge case can rest with it's mouth properly headspaced forward against the chamber ridge where it belongs, but with considerable potential free movement behind the cartridge base along the toggle train.

With even a few thousandths of free play behind the cartridge base and forward breech block, when the cartridge is fired, high pressure will drive the entire toggle train back over the free space hard until the toggle train is compressed. At that point it will abruptly stop and shock will cause a high pressure wave within and across the toggle train.

Eventually, hardened and brittle areas of the train (in my case I lost a forward axle pin repeatedly) will snap or crack. The fix was to resolve the problem by eliminating the forward / rearward play. I fixed my issue with an oversize axle pin and resized hole in the breech block.