Slug?

I don't know much about ID-ing finds (yet) and even less about bullets. This is a shot bullet, right? It's just under 2cm. One side is relatively flat (I thought it was a button originally), the other has a concave notch sticking up out of it.

Is it ever possible to determine what a bullet might have been from when shot, and possibly guess the age? Is the shape from the impact of hitting the ground, or something else? Thanks in advance for the lesson!