Many times when I'm asked to ID a "shell fragment," it unfortunately turns out to be a piece of broken iron pot or waterpipe. Happily, this time I'm able to give good news... it IS a fragment of an explosive cannonball.
"Precise" measurements would help, but based on size comparison with the US quarter-dollar in the photos, it appears to be a fragment from a 12-pounder caliber (4.62-inch) cannonball.
And, assuming you are hunting a civil war era battlefield, it very likely is a Confederate cannonball fragment, based on the type of fuzehole in it. Your shell-frag's fuzehole lacks threading for a screw-in fuze, which means it held a wooden fuzeplug. Very-very few yankee 12-pounder cannonballs had a wooden fuzeplug,
More specifically, your frag is from what was called a Case-Shot shell -- which meant the shell contained antipersonnel balls, in addition to the usual blackpowder bursting-charge. A shell which contained only gunpowder was called a Common-Shell {"common" meant plain/ordinary shell).
For 12-pounder caliber cannonballs, the shellwall-thickness of a Common-Shell was about .7-inch. For the 12-pdr. Case-Shot version, the shellwall-thickness was about .45-inch... which appears to be the thickness of your 12cannonball fragment. The difference in shellwall-thickness can be clearly seen in photos below, showing a sawed-in-half 12-pounder Common-Shell and a 12-pounder Case-Shot (with the antipersonnel balls inside it).
Pardon me please, but I must contradict a previous poster's guess -- IF you dug it in Kentucky, it is not a Coehorn (mortar) shell-fragment. The nearest place to Kentucky that any 12-pounder caliber Coehorns were used was at siege of Vicksburg Mississppi.
Knowing you are new to the metal-detecting hobby, and again assuming you are hunting a civil war era battlefield, I'll say congratulations on finding your first civil war artillery shell fragment.
