cannon ball ID

[adrian]

Would a small sphere such as mine more likely be a grenade or a very small mortar shell? Did they make them this small??

The smallest mortar in British service was the Coehorn at 4 2/5 " bore.

However that does not help differentiate between a shell or grenade, a 4pr cannon could fire a shell, as could a 3 pr etc. I would suggest that shells were often used as grenades - hence the British (apologies to the rest of Europe etc for using them as an example all the time) had 3pr grenades (same dia as a 3pr common shell) & 6pr grenades (same dia as 6pr common shell) & eventually grenades were purpose made. Just as solid shot were used for the sport of shot putting, eventually shot puts were purpose made.
(but sometimes still found on e-bay being sold as antique cannon balls....)

In summary, its quite likely that what you have could be both, unless you know its country of origin & investigate their cannon calibres which might narrow that probability down if it is not of a cannon caliber.