3 inch ordinance for a rifled cannon. it has beautifully preserved rifling grooves in it.
considering where melrose, fl is located, there is a very good chance that this is civil war era. you are situated between 2 battles. you had fighting on the st. john's river near palatka and you had the battle of gainsville. you also had raids all up and down what is today hwy 301. waldo, starke, baldwin. not sure if there was any fighting in melrose though.