I think this has been covered before on the forum— the Roman siege of Burnswark Hill, a fort in Scotland.

The original publication is by G. Jobey, in Transactions of the Dumfriesshire and Galloway Natural History and Antiquarian Society 53 (1977-8)-- the lead sling bullets are either shaped like literal acorns (glandes in Latin) or the more normal "rugby ball" model. Here's a picture.