Hallo lepidina and welcome,
If I remeber corectly, the ancient Romans used missiles of weight about 20 g. I'm also wondering, if they would be effective enough during battles. Most probably lead missiles would be, but clay ones? One of the forum members, Hondero from Spain, collects oryginal sling projectiles. For sure, he could say much more about them.
Anyway, from my limited experience with very light (20-30g) stones, and light sling, I know, they can fly very quick and far. When they are good shaped, for sure, an experienced in range shoting slinger can send them relatively easily at ranges above 250 m, saying very carefully. Clay missiles, for sure, can fly a bit worse. At my guess, they are able to surpass ranges above 200 m, assmuming, that a light and a rather thin (more aerodynamical) sling is used. Anyway, tests are necessary, for more accurate numbers. Hope, it helps a bit

Jurek