AFAIK sling bullets tend to have a relatively level trajectory, and are thus rather ill-suited for any kind of "close fire support"; for that kind of thing archers work better, and dudes with javelins even more. AFAIK it was perfectly normal in late-period Roman warfare at least for the rear-rankers to carry multiple light javelins and make themselves useful by lobbing those into the ranks of the enemy infantry behind the clashing first-rankers - though, usually the enemy was returning the favour...