Thanks. This one is stuck very solidly, as if it was welded, as I can tell from the ring when the rod strikes the anvil, which is in contact with the breech door. It is always hard for me to believe that only rust is acting like a weld, but I'm sure that's the case here, there's nothing in the design of this thing that would allow the breech plug to hang up mechanically and not want to pivot back and down on the breech door hinge.
I wasn't sure before I started that this one was made to take metallic cartridge cases, but so far it seems that it does. The chamber area is noticeably larger than the bore. Also the bore isn't sealed by the breech door, since penetrant leaks through pretty freely from the bore to the outside in the area of the hinged breech door. Morratt's first b/l cannon, the one shown in his oriiginal patent, did not take cartridges, and all three times he took his gun for testing at the Navy Yard, there were gas leakage problems. I'm sure that's why he decided to go with metallic cartridges in his three subsequent guns, namely the two howitzers (3.4 in. rifle and 12 pdr. smoothbore) and his Army 6" rifled howitzer.