Look at the position of the top center chamber and see how it appears to be out of alignment with the forcing cone. I thought perhaps it may be that whoever loaded the weapon did not allow the chamber to "index" properly; that is the firing chamber and the barrel must be perfectly aligned in order for the projectile to enter the barrel properly and allow the rapidly burning/expanding gases to vent quickly. The cylinder stop tab usually accomplishes this purpose.
If the firearm was operating properly I don't see how this could happen. The internal mechanism of the revolver works to prevent any misalignment. As the trigger is pulled (double action) the cylinder is rotated and locked in alignment prior to firing. The same thing happens in single action.
Of course with wear the chamber can get slightly out of alignment with the forcing cone. There are several reasons for this. But when it happens, a small portion of the projectile may be "shaved" and ejected out the side of the weapon between the forcing cone and the cylinder.
I experienced this in the police academy when we trained on 30 year old revolvers (Colt OP's)that had a bit of "slop" in the cylinder alignment. Splinters of lead shot out of the revolver being fired in the station next to mine and embedded themselves in the back of my left hand.
It stung a bit.
From the looks of what remains of the chamber wall, this weapon was more than just a little out of alignment. Yet it does not appear to be that old.
Anyone have thoughts on this?