Mung beans are good to start with. They too will break down over time and produce a powder, as you gradually hit the bag with more power. The Chinese believe that the powder from crushed mung beans can help nourish the hands. Over time you can move on to gravel and eventually iron pellets, but it's best to wait for your teacher's advice. Iron palm training without proper guidance can lead to irreversable damage.
Air soft pellets make a perfect filler for punching bags.
I guess a kidnapped midget is out of the question....
My guess would be sand.
I know rice is a bad material because it breaks down into dust overtime. I am a intermediate kung fu man (ranging from novice, intermediate and advanced.) I do not know what materials to use I want to improve my kicks and learn the iron palm technique. Any suggestions please?
BQ: What is a "filler"?