sling vs bow

[D B Campbell]

Maybe what is being described are clay projectiles [i.e. pots] filled with some sort of burning substance.

I agree that pots would be more sensible, but Caesar definitely refers to glandes (i.e. bullets).

Slingstones covered with clay to create a "frag" effect have benn found in Sesklo Greece and Middle East.

That's interesting, Stefanos. Are they published somewhere?

When I explored Hatra in 1984, amongst dozens of catapult stones I came across one (about the size of an orange) that had been coated in something that looked like bitumen. The coating had mostly flaked off, but I would estimate the original thickness at about 1cm. At the time, I wondered if it might've been some kind of incendiary missile.