[Sender]

What did these cannonballs use for a detonator? Was there some kind of primer/fuse that went off on impact?

[AndrewB]

Frequently a cloth or fiber fuse that screwed into the hollow ball like a plug. Ignited by firing the gun. Could be cut to time the detonation to occur in the air over enemy troops.

BTW, the article mentions "rusty bullets"- Not possible.

[Jimmy Valentine]

The cannon balls used in the field were hollow and filled with gunpowder. The fuse was ignited by the muzzle blast of the cannon. The shell did its damage by exploding and the shrapnel is what killed the enemy. This was not a totally precise method and there were a lot of air bursts and duds in the mix.

Looks like this collector was working on a dud that decided that it was time.