Posting for a friend who found this in her yard. The park across from her yard was a Civil War camp site. It looks modern to me but wanted to get expert opinions.
Nokta FoRs Gold, a Gold Cube, 2 Keene Sluices and Lord only knows how many pans....not to mention a load of other gear my wife still doesn't know about!
Smokey is correct. The multiple tiny parallel ridges inside your bullet's body grooves first start showing up about 1877... and did not become "commonplace" on bullets until the very-early 20th-Century. Those parallel ridges (very similar to the edges of US dimes, quarters, halves, and $1-coins) are called a "reeded body-groove" or a knurled cannelure.
There is a chance your bullet was made in the late-1800s, but annual statistics on bullet production indicate your find was (by far) most likely made sometime in the early 20th-Century.
Identifying it correctly without precise measurement of its diameter is difficult. If I had to guess, I'd say it is an early-1900s bullet for the Indian Wars era US Army .45 Colt metallic-cartridge revolver. (The civil war version of that pistol was 44-caliber, not .45.)