Fired and unfired pistol, rifle and shotgun ammo. I think I could give an interesting talk on the differences and the history of various cartridges, no fine details, only a broad overview. To keep it fun, I'd sprinkle in lots of things most people probably don't know.
For example, did you know an AR-15 round is actually kinda wimpy, especially compared to a typical hunting round? In some states, ARs are illegal to hunt with because the bullet is deemed too small to make a clean kill. It's basically a hauling ass .22.
And .22s are a neat topic by themselves. There's crazy variety out there, even tracers!