I was wondering whether there were caps on both sides of the bullets serving as spikes, having a similar effect as arrowheds. I think not
.
Firstly the question presupposes that an ancient lead bullet would have a stable point-first flight. The jury is still out on that issue, but I am inclined to thing that this is unlikely.
I have seen a good many ancient sling bullets and there are no traces of iron. Although iron could (an probably would) corrode away to nothing, I would expect traces of iron to leave a stain and there is no sign of this.
An iron jacket would be expensive and time consuming to make. A simple spike (a nail) embedded in the lead would be much cheaper, simpler and possibly more effective. But this would be clear from the archaeological record, but there is no sign of such finds. In any event, by the time one has designed a bullet like this, one might as well be using the dart throwing cestrosfendon. Of course a dart
does ensure point first flight.
See here:
http://www.slinging.org/forum2/yabb/YaBB.cgi?board=1;action=display;num=10747134....
In the ancient world all metals were valuable, but lead was by far the cheapest and easiest to work; it was the plastic of the age. I suppose it is possible that solid iron bullets could have been produced and these being lost from the archaeological record due to corrosion, but even this seems unlikely.