Hey Gary, without even looking at it, I am willing to bet you that the unconventional shape of the chamber (i.e., Cylindrical) was the problem here. Think about it. Looking at reply #184, you can see how small the investment entry hole is. If you decide to improve this condition, a simple change in the shape of your wax drilling, spade drill to a Gomer Chamber shape would allow easier entry for the investment material, because a cone shape with the large end up would facilitate it. See the line drawings below for a clear comparison of the two styles of chamber.
I'm speculating here, but I would almost bet that you will find solid bronze below the point of intersection between the Ball Seat and the Chamber Mouth. This means that there was an air bubble below that point, when there should have been solid investment material.
Tracy
The cylindrical shape you have. These section drawings are from Gibbon's Artillery Manual. We bought ours in 1973 and it's a great addition to any artilleryman's library.
The traditional Gomer shape, (conical) which the original Coehorn and many other mortars had for easier loading and fewer cracked and disintegrated shells.