Not sure why my text didn't show in the first post.
This was dug at a known Civil War training camp/firing range in mid-state NY. It's 27 mm in length and 11-13 mm wide (call it 12 mm). When dug it had a nice whitish patina, but it accidentally made it to my rock tumbler for clad finds (oops), hence the newish gray color. Any help would be appreciated. Thanks.
It appears to be a civil war era .54-caliber US 3-groove Minie-bullet -- unfired, but in rather beat-up condition. Unfired .54-caliber muzzleloader rifle bullets typically measured .52 to .53-inch in diameter, which matches your beat-up bullet's approximate measurement of 12mm to 13mm in diameter.