For anybody here who doesn't already know:
A "copper-jacketed" bullet means the lead bullet's body is partially or entirely covered by a thin shell of copper. Brass-jacketed and steel-jacketed versions also exist. Made for use in repeating firearms, to keep slivers from the lead bullet from building up in the gunbarrel's rifling grooves, which would eventually clog the grooves. First developed in the very late 1800s, and still in use today. So when you see the copper (or brass or steel) jacket on a bullet, you'll know that the youngest it could be is several decades after the end of the civil war, and most likely from sometime in the 20th Century.
One photo below shows a sawed-in-half copper jacketed bullet with gunpowder in a brass cartridge, on the left in the photo. The other photo shows some "hollow point" copper-jacketed bullets.