It is a very rare variety of Sharps .38-caliber Sporting Rifle bullet, from the 1850s through 1860s. Collectors call that specific variety a "Multigroove Sharps." But in actuality, your bullet's distinctive form was not created by Mr. Sharps. It was invented and patented by Gomez & Mills, US Patent #21,253, dated August 24, 1858. Although as the rifle's name Sporting Rifle indicates, it was sold for Civilian use, your bullet is considered a civil war bullet, because a significant number have been dug from civil war Military campsites and battlesites. Some soldiers brought their own rifle when they entered the army. I should mention that your bullet is found primarily in yankee army sites, but the Confederates also had a few of them.
Was there any civil war Military activity at the site where you dug your Gomez & Mills bullet?