According to Vetron's Guide to Weapons and Armour, the sling has a "2" effectiveness rating.
Slings first came into use around the time of Ultima III. In Ultima IV, a few prudent magi first took to using them as handy backup weapons. From Ultima V onward, they changed little, although properly-shaped ammunition became somewhat less common after Ultima VI.
The common sling is fashioned from twenty-four inches of cloth, folded in half, with a leather cup. The projectiles — small rocks — are easily collected, making this an inexpensive missile weapon. A perennial favorite with travellers down on their luck.