All war gamers that use Carthaginian armies know about Balearic Slingers. So this type of infantry requires mention especially since we're concentrating on Iberian armies. Their skill with slings meant the indigenous peoples of the Balearic Isles were much sought after as mercenaries and would have been part of the vast majority of Spanish armies.

The legend of the boy children not being allowed to eat any food they could not hit with a slingshot is well known, but interestingly they were not necessarily the best slingers on the battlefield, being forced by poverty to use stones, while the Rhodians and other Greek professionals used cast lead shot. However, they were very good, if raggedly dressed, troops. Armament was usually a sling, a knife and a small shield, which was strapped to the forearm. These troops would not have closed to combat if at all possible, acting only as fire support.

Prior to use by the Carthaginians the islanders dressed in animal skins plying their national weapon while some used fire hardened sticks as javelins. They are reputed to have used up to three slings, the spares being wrapped around the head. Prior to the birth of tourism, the only viable export from these islands was their slingers, and armies that tried to take over the island received a missile-rich welcome. In addition the lack of any exploitable resource on the islands made them an unattractive prospect for conquest.

In formal battles these troops would usually open proceedings with showers of slung stones, which were said to be more effective against armour because of their concussive effect. As the lines closed they would draw off to the flanks, firing as they went. In Carthaginian armies they would also operate in support of parties of javelin armed lights. Strabo - 3.5.1 (Vol II, p 127 of the Loeb edition). Livy (various)