It is definitely a bullet. Using TreasreNet's click-twice-for-supermagnification option, its sides show axial linear rifling-marks from the gunbarrel's rifling grooves.
Specific info about your bullet:
1- It has at least 7 (possibly 8) body-grooves.
2- It has a flat solid base.
3- Combining characteristics #1 and #2 above, it is a 20th-century bullet, because no 19th-century bullets had 7 (or 8) body-grooves with a flat solid base.
4- Comparing your bullet's length and diameter with the quarter in the photo, it is a large-caliber rifle bullet, not a pistol bullet.