Your bullet is an unfired civil war "Minie ball." There are many-many versions of Minie bullets. Your version is a yankee "generic" 3-groove Minie. Relic-diggers typically call those simply a "3-ringer" but in actuality it has grooves... not rings, which are raised bands. (For example, a Sharps bullet has rings.)
The nose-tip on yours is sharply pointed, which is unusual on .577 and .58-caliber 3-groove minies, so I think yours is a .69-caliber bullet.
Here's how you can tell its caliber without measuring:
An unfired .58-caliber Minie's diameter is approximately .56 to .57-inch.
An unfired .69-caliber Minie's diameter is "approximately" .67 to .68-inch.
A US dime is a hair larger than .70-inch.
Put your Minie's base onto a dime.
If its base is only VERY SLIGHTLY smaller than the dime, it is a .69-caliber. If it is 1/8-inch smaller than the dime, it is a .58-caliber Minie.
Please do that size-comparison test, and tell us the result.