My Bronze shaft Mortar Project reached an impass...  my limited equipment & skills mandated outside help.  I sent my pieces...  to a skilled metalworker.   Upon reciept, he evaluated my pieces & my "make do" plans & suggested that there was a better way to proceed.  He suggested turning the breech to a true hemisphere & using a matching ball end mill to machine the trunion.  I readily agreed.  He was kind enough to provide me with periodic progress pictures:

Before

After cutting Hemisphere 

He made a fixture to hold the trunion shaft

Milling the trunion

The trunion pocket

Test fit the trunion on the barrel.  Flawless machine work & perfect fit.  
This is just a huge step up from what I had envisioned looking at the original cut off with an existing cut (which I eliminated in my initial breech turning.)   I shipped out salvaged junk & got a Mortar back!  The vision to look into a piece & see what it can become is priceless.  

Unanticipated Problem!  The welding gave him fits...  the unknown alloy just did not want to weld.  I hope we are still friends after this.  I had no idea it would be a bow-wow!  
Shipped 2 days before Christmas (flat rate, if it fits, it ships!)  Amazingly, I recieved it 2 days later, after noon on Christmas eve.  I will dress the weld a bit after I recover from Christmas.   See there is a Santa Claus!