A sling bullet is a defunct object that formerly appeared in NetHack until version 3.0.0, and was used as ammunition for slings. Sling bullets dealt 1d6 + 1 damage versus small monsters, and 1d4 + 1 versus large monsters, the same as crossbow bolts.
The early versions of hack121 and PDP-11, variants of Jay Fenlason's Hack, used various types of sling bullets - interestingly, rocks did not exist as objects until the first version of Andries Brouwer's Hack. From these versions to Hack 1.0.3, sling bullets had their base damage values against large and small monsters reversed. As the weapons and armor are derived from Dungeons & Dragons, this reversal was likely unintentional - NetHack 1.3d sets the damage values to match that of the source material.
Sling bullets and their damage values are derived from the Advanced Edition of Dungeons & Dragons, where they dealt 1d4 base damage.
As the sling is a widespread weapon dating back to Neolithic cultures (if not still-older ones), sling-bullets and similar objects used as sling ammunition are common archeological finds. The size of a sling-bullet varies greatly from no larger than a mere pebble to fist-sized stones weighing a pound or more, and in the hands of a skilled slinger could be hurled upwards of 450 meters (1500 feet) at speeds over 250 miles per hour.
Rocks used as sling ammunition are frequently salvaged from river depths, where the rushing water would slowly shape the stone round. Bullets are also purpose-made from various materials such as clay - in Classical times, lead-cast bullets were considered the most efficient and best-quality ammunition. Symbols such as stylized lightning bolts, snakes and scorpions have often been found molded into lead sling-bullets from around this time; others have had holes drilled into them that produce a whooshing sound in flight. Slinging as a sport is still practiced in the Balearic Islands (where slinging is a tradition) and elsewhere, and slings also see use as weapons for hobby purposes.
Sling bullets return as items in the variant EvilHack, introduced in version 0.7.1. Their default object material is iron, and they appear as shiny stones when unidentified. The damage they deal was changed to 1d6 vs small monsters and 1d8 vs large monsters. Cavepeople of all types can start with sling bullets.