Double D - I will be more careful in future to follow pm requests and other forum protocol. Sorry for the mistake. I'm brand new here as you know.
To answer your questions:
I will address your questions generally, first, and then more specifically.
Molten metals shrink on cooling for obvious physical reasons.
Accuracy then, from casting to casting, will depend upon mass or cross-section of the casting. More mass equals more shrinkage. The metal may not shrink uniformly, especially when the casting exceeds whatever the mass limit might be for the particular alloy. Some alloys shrink or deform less than others and so if you want to cast small cannon balls, for example, it would be good to consult a professional metallurgist for advice which alloy to use. From there, you can get the alloy and start casting. Shrinkage can be minimized and sometimes controlled or prevented via fast quenching of cast pieces. Possibly makers of musket balls in the old days, cast them and quenched immediately in cold water to keep diameters to accepted limits. Of course, a cannon ball could be cast larger than needed and allowed to shrink down to target size as well. The smaller your ball, the less affect shrinkage will have.
Since my purposes have always been art or sculpture oriented, I would say we'd have to do some small experimentation to find out whether the casting process I am familiar with would work for cannon balls. My guess is that up to 1/2 inch or 3/4 inch, possibly a full inch, accuracy and repeatability would be excellent and doable. Beyond those sizes we will start getting into shrinkage issues and have to become creative to prevent or overcome it via quenching etc.
