Taz42o is correct, as usual -- your iron ball is definitely a Sports Shot Put.
To read my professional qualifications for making the following statements-of-fact, please go to my profile page and click "About me."
Absoluitely no historical cannonballs were marked with their weight, nor their diameter-size. The cannon's crew doesn't need to know the ball's weight, but Sports-contestants do.
Taz is also correct that some (but not all) Sport Shot Put balls have a screw-in plug to allow small lead balls (or sand) to be added into them, to make minor adjustments to their exact weight. Reason: For the purpose of fairness in the sports competetion, each contestant's ball must weigh precisely the same as those of the other teams'.
Regarding your concern about a cannonball still being "live":
It is true that certain types of artillery projectiles manufactured during the 20th-Century can still be "shock-sensitive" -- meaning, they can explode from merely being dropped. However, that is not true about antique cannonballs.
Civil war relic diggers are notorious for cleaning the rust/dirt concretion off of their cannonballs by bashing on them with a hammer. Furthermore, almost every excavated (dug-up) civil war shell got struck by the shovel during digging. Not a single explosion has resulted from hammering or shovel-whacking an excavated civil war era shell. So, you do not need to worry about dropping a pre-1890s artillery projectile.
All of that being said... some excavated pre-1890s artillery shells can contain unspoiled powder -- which a very hot fire or drilling into the shell can cause to explode. So, as long as you don't put them in a fire or drill into them, you do not need to worry about handling pre-1890s artillery shells.
As Taz42o indicated, pre-1890s (Indian Wars, Civil War, War-Of-1812, Revolutionary War, and Colonial-era) artillery projectiles are my specialty-area of relic study. Post photos of the ones you've found if you want me to give you specific ID for them.
