I have now found (by chance looking for something else as always on the WWW)
a reference to bronze cannon firing iron balls from Florence in 1326. In
1350 Petrarch was writing of "These instruments that discharge balls of
metal....are become as common..." This , of course could be lead but lead is
not good at knocking down walls, which was surely the primary use of early
cannons. By 1377 at the seige of Odruik cannon firing 200lb balls breached
the walls. That would be a big stone. There are references in the WWW
article if you are interested.