The thing with slingers and the original steppe horse archers is that the stupid amounts of training time were developed in boyhood/young adulthood through hunting. They weren’t proficient in their weapons as a result of explicit military training, more the military application was an outgrowth of the proficiency needed for survival.
In the Inca Empire for example all boys aged between 8 and 14 were sent into the fields with slings to keep birds off the crops. As a result the Inca army was full of expert slingers. There are Spanish reports of Inca soldiers being able to hit raised swords with sling stones from long distance.