Larry and Chris, I agree with you that the light projectiles have bad behavior in flight, because they are restrained by the air in greater proportion than heavier ones. Although I´m not an expert in range, the projectile I prefer for reach is of about 50 gr., close to the 2 oz present record. Nevertheless the historical data are something differents.
From the distant times of the Neolithic, when the sling began to be used military, the more or less standardized projectiles had an average weight of 35 grams. In later times the clay projectiles were continued using everywhere. The Carthaginians used them and also the mercenary troops of different origin to the service of Rome. Then, the average weight of the projectiles continues being around that weight of 35 grams. The revolution caused by lead glandes invented by the Greeks and adopted immediately by the Romans, gave rise to a very diverse manufacture of projectiles according the function wished for them, being used sometimes projectiles of around 100 grams, but the average weight, according to my investigations between different authors place them around the 40 grams. There are authors than place the average weight lower, about 30 grams. Everything depends on the examined archaelogical collections. I have a collection of 350 glandes and the average weight is 50 grams approximately, but there is a significant proportion of them of small size, of around 20 grams. It is necessary to think that these small projectiles would have been used specifically to obtain reach, because at medium ranges it would be preferred to use projectiles of greater power of impact. All this considering that the light old slings would have an approximated weight of 20-30 grams., very close to the weight of the proyectil, which was not the most advantageous condition to use this type of light projectiles. With the modern materials lighter slings can be constructed that optimize the use of these small projectiles. The experimentation is fundamental in this subject, since mathematical calcules are very complex and not much adequate to the speeds that are handled with the sling.
Dan, I didn´t know this reference, it´s nice to find a new one