I found part of a cannon ball when I was about 6 or 7.
Problem is I can’t find were there was ever a cannon that size in the state of Texas.
The cannon would have about 10 inch bore. That’s a massive cannon and would have only been used on a ship, not on land.
Siege mortars were plifered from Federal armories during the Wah of Northun Agresshun. 8 inch, 10 inch, 13 inch...perhaps a relic from one of these?
The 12 inch Columbiad was available in those days. A giant among field pieces, usually used in shore batteries along the ocean or navigable rivers. http://en.wikipedia.org/wiki/Columbiad
There were land-based cannon and mortars used in the Civil War that had 10-inch and larger bores