Definitely not a cannonball. The "walls" of an artillery roundshell have to be quite thick, in order to be able to withstand the blast of firing, which will shatter thin iron.
By the way... the bigger the shell, the bigger the cannon's propellant powder-charge needed to be. So, the "shellwall" of bigger shells had to be thicker than smaller shells.
If anybody wants to know... the thinnest shellwall for a cannonball was in the 6-pounder (3.67"-caliber) antipersonnel shell (known as a "Case-Shot" shell). Its shellwall was about .40-inch thick. For a 6-pounder Common-Shell (no antipersonnel balls inside it), the shellwall thickness was about .60-inch.
Speaking of thickness... a crucible's walls are significantly thicker than a cast-iron pot's walls. You'll need to clean the rust-crust off of your find and measure its thickness to know for sure. To me, it seems like without the rust-crust, your find is about the right thickness for an iron pot.