Only you can tell us the caliber, nobody in the world can tell caliber from a photo. You need to get a caliper and measure the diameter, you can get a cheap digital caliper for fifteen bucks. It is not a minie ball, a minie MUST have a deeply hollow base so the thinner bottom expands into the rifling of the firearm where the grooves or "rings" are. A good in-focus closeup of the side would help date it, if there are little lines in the grooves it is a more modern slug.