You might want to clarify some of your terms here because as it stands this question seems confusing.

"Ancient" would seem to imply you're talking about pre-fall-of-Rome so not really a time when people would be using cannonballs. I'm guessing "in the middle" means "in the Middle Ages" which does stray into the world of cannonballs but that's very much at the other end of the period.

It might also be worth clarifying what you mean by "Africans". North Africans were in pretty constant contact with Europe because North Africa is right on the Mediterranean but you might really be trying to ask specifically about sub-Saharan Africans. [Edit] although as the linked thread points out, that distinction is arbitrary, perhaps to the point of being unsupportable.

You could start by looking at this thread by u/cleopatra_philopater which talks specifically about contact between Rome and various African cultures.