Sorry to have to disappoint, but it is definitely not a fired Whitworth bullet.
1- Whitworth bullets were .45-caliber. The ruler in your photo shows your bullet's body is at least .50" wide.
2- The Whitworth rifle's bore is hexagonal, instead of cylindrical with typical rifling-grooves. Therefore, fired Whitworth bullets show six "flats" encircling the bullet. Your fired bullet shows none of those "flats" on its sides.
Your bullet is a fired .54-caliber "Enfield Pattern" bullet, which was fired from a .54 Austrian or .54 Springfield "Mississippi" rifle.
Additional observations:
1- The bore of the rifle which fired your bullet was so clogged with powder-ash (from repeated firing without having the spare time needed for cleaning the bore) that the soldier had to "hard-ram" the bullet. Hard-ramming causes the rifle's ramrod to make an imprint of its circular lip in the soft lead bullet's nose... as we see on your bullet.
2- The buildup of powder-ash had also clogged up the bore's rifling-grooves... which is why there are no rifling-marks on your fired bullet's body.
The bullet on the right in the photo below is another example of a fired "hard-rammed" .54 Enfield Pattern bullet.
