Need precise measurement of your bullet's diameter, in hundredths-of-an-inch, because even as little as .02-inch can make a difference in a bullet's ID. You'll need to buy or borrow a Digital Caliper to do measuring that precisely.
Also need photos showing the bullet's top and base.
With a flat base and being a hollow point my first guess (not knowing the size of your thumb) would be that it is a 16 gauge shotgun slug that was fired through a rifled slug barrel. As CannonballGuy said, we'd need precise measurements to id it, and a weight in grains as well would be helpful if possible.