Tony is right, mostly. It is a copper-jacketed (not "plated") hollowpoint bullet from sometime in the 20th Century. The copper jacket is manufactured separately from the bullet's lead body, and is MUCH thicker than plating. Here's a photo showing a sawed-in-half copperjacketed bullet, on the left, but that one not a hollowpoint. The other photos shows the hollowpoint version.
Although you say your bullet's diameter is 1/2-inch, its body is too short to be a .50-caliber US Machine Gun bullet... so it is probably a .45-caliber.