In games, overlapping advantages at a shared place and time create timings of opportunity. In Dungeon Crawl Stone Soup, halfling characters often take advantage of a powerful early sling timing, due to several static game elements that favor early sling use.
Experience points (xp) are earned by killing monsters and spent to increase skill rank. Each rank in a skill costs more than the last. Further, character species vary in skill aptitudes, which determine the efficiency with which a character spends xp into skills. Halflings have an impressive +4 aptitude for the slings skill, which doubles all xp spent on slings. Because lower levels in skills are cheaper, and halflings have such a high slings aptitude, it can be very cheap and effective (in terms of xp) for a halfling character to invest a bit in slings early. The early ranged attack cheaply kills melee enemies before they close on you, and enables fighting ranged enemies without needing to close oneself. A character with less aptitude for their early weapon skill, when later considering a transition to a different weapon skill, would be more pained by abandoning the sunk xp cost:Non-sling ranged characters may be limited by ammo in the early game, and often should conserve ammo by using melee attacks. However, sling ammo is very common, with the weakest type (rocks) being essentially limitless.
Early game halflings with slings use ranged attacks at the lowest cost of any shooting character, in terms of both ammo and xp.