Just to share information...as this is a first for me, and first I've heard of in my (limited) experiences.
Yesterday, I had 44-40 cartridge separate upon firing. The cartridge broke cleanly at the start of the bottle-neck. This section of the cartridge went down the barrel, and made it to within about 3 or 4" from the muzzle. This was a BP cartridge with a typical load of this cartridge - ~32 grains of 2F Goex.
I noticed nothing unusual about the shot fired from this cartridge. I was at the range and shooting at 50 yards, so I didn't realize (at the time of the shot) that it didn't even come close to the target.
Though nothing serious may have occurred with a shot following this event, I wouldn't want to find out for sure. Several coincidences kept me from taking another shot. It was the last shot in a string of 10, and I was cleaning the barrel between shooting strings. I didn't eject the cartridge to the ground, but grabbed it before it hit the ground. (Typically, I eject, let the cartridge fall to the ground and pick up my brass at some point later.) So, I realized that the cartridge had broken....but I expect the neck to be in the chamber not the barrel! I was able to force the broken section out with a cleaning rod and jag.
A few things I learned.....
Inspect cartridges thoroughly before reloading! As a general rule, I normally reload my 44-40 cases until it shows a split at the mouth on the expansion step. Obviously, in retrospect, this cartridge should not have been reloaded. It appear very tarnished and brittle!
Check neck sizing die...
I discussed this with a few other reloaders, and one pointed out the I could be "oversizing" the neck leaving a gap in the chamber...that is, I'm moving the profile of the bottleneck further down from the top of the case.