cannonball ?

[adrian]

It is most likely a cannon ball. The mould was in two parts & where they join a seam can be seen - which is why you have what appears to be two parts joined, but in fact is one solid piece. The small circle is the part were there was an opening in the mould for molten metal to be poured in, this is usually called the "sprue", the sprue metal is ground down level with the circumference but is usually still visible, as on this example. Some balls have two sprues, opposite each other, these have come from a "gang mould" whereby more than one ball is cast from a mould.
Adrian