The other post is wrong, as far as I understand - the word cannonball refers to things that weren't rockets, grapeshot, or exotics like carcasses. However, there are 'shot' cannonballs and 'shell' cannonballs. The Huolongjing, a famous 14th century Chinese treatise, references both, specifically talking about round, hollow metal shells that would be filled with gunpowder, and (IIRC) the context was about use in cannons, not just trebuchets or the like.

I don't think they were used in Europe for some time, but Chingis Khan was using shells on open fields and during siege.

It's a ball, and it goes in a cannon, and I believe that qualifies to be a cannonball.

So, were there explosive cannonballs? Yes. In naval warfare? Yes! Here, after just a cursory search, is an account of the adoption of shell-firing guns. For a tl;dr:, the book represents naval minds strongly cautioning against the use of shell guns, because they were relatively low-range - they couldn't pack too much of a punch because that would detonate the shell - and the shells themselves were unreliable. Shot, on the other hand, is quick, safe, and accurate - the explosive power didn't mean so much when you could just tear through wooden decks with sheer momentum.

EDIT: And there is a serious difference between a carcass and a shell. A carcass is a lit incendiary, meant to spill open when it lands. A shell is an explosive, a gunpowder weapon.

The word "shrapnel" comes from an English officer named Shrapnel. He invented a shell with a fuse that would explode some time after it was fired, spewing small metal balls at the enemy. Wikipedia has the first use as 1804.

http://en.wikipedia.org/wiki/Shrapnel_shell

The US national anthem mentions "the bombs bursting in air" during the British attack on Ft. McHenry in 1814. This was a naval bombardment where exploding shells were used. However this type of shell was fired from a short barrel mortar or howitzer, at a high trajectory to the target.

One needed the high trajectory to allow time to light the fuse, load the gun, then shoot it before the fuse blew. The naval canon which shot a flat trajectory had too short a flight of its shell, making an exploding shell dangerous to the gun crew.

In the mid 1800s a system was developed to arm the shells as they were fired, allowing their use in cannon.

Can someone reference civil war cannonballs? I was under the impression that round, solid iron balls (i.e. "cannonballs") were a very viable and common weapon in civil war battles because when fired at massed infantry, you would essentially be taking out several soldiers as the cannonball passed through a group of them.

And I believe these balls are still around, stacked in pyramid shapes at various museums, monuments, etc (and presumably not packed with explosives).

Thanks to the OP for this very interesting question!

Okay, I'll try to give you a succinct answer, because some of the answers in this thread are all over the place.

In the era in question, all firearms fired round projectiles. The reason for this is because rifling was, due to the powder and materials used, unfeasible for practical use. A cylindrical projectile without spin will wobble out of control, while a round projectile will at least simply turn on its own axis.

So, the problem with round projectiles is that you can never know with which end it will hit, which makes the idea of percussion fuse untenable. Instead you're left with two choices: either solid shot or a timed fuse attached to a shell. In the general sense, cannon would fire solid shot at high speed in a straight trajectory while mortars would fire shells, which due to their fragility would be fired at lower speed in a ballistic trajectory. During the 18th century a compromise between the two, the howitzer, began seeing use: a indirect fire weapon firing at a slight angle, firing shells. Now, indirect fire is great if your target isn't moving about, like a fort, but kind of useless otherwise, because the art of ballistic calculation is still in its infancy, and sighting has to be played by ear.

So, to return to your question: infantry and ships were both moving targets at relatively short range, so direct fire was by all means preferable to indirect fire: hence both ships and land armies both used solid shot (ignoring grape and canister for the moment). A cannon ball striking a ship wouldn't explode, but would strike through the hull causing damage and splinters as it went, while round shot fired at infantry would bounce along the ground knocking off limbs as they went.

Lastly, not that I would ever defend an atrocious movie like The Patriot, but in Mel's defense I don't think it ever showed a cannon ball exploding.

As an incidental answer to the second part of your question, "were they just balls of metal?" I can tell you that during the English Civil War, iron balls were not universally used on land. Stone cannonballs were often employed, despite being less penetrative or powerful, as they were cheaper and easier to provide.