Stuck Live Round

[SMM123]

In your first picture of the inside of the upper receiver/barrel extension, there is something at about 4 o'clock relative to the barrel extension. Is that dirt, a crack, manufacturing flaw, or just an unknown bit of junk in the receiver?

If it is a crack, I would wonder if it contributes to any of the phenomenon you describe.

Most of what can be seen in the chamber area is propellant and other residues, left from when the case separated from the bullet during mortaring. There are no cracks.