HuntinDog is correct... it is definitely a copper "jacket" from a 20th-Century bullet, missing that bullet's lead core.
Here's a simple solution, for relic diggers to keep in mind:
The diameter of the cylindrical portion of a musket/rifle percussion cap is exactly .25-inch. There are no copper-jacketed bullets which are that small.
I should mention... note that I said "musket/rifle" in the statement above... pistol percussion caps are even smaller than .25-inch.