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Use beans are used inside the bag. Traditionally, mung beans or soy beans, but I suppose other types of beans can be used as long as they aren't very fragile. Not sand as you've found out they will seep through the spaces between the canvas fibers. Another alternative you can use are the plastic pellets used in airsoft guns work pretty well also . You need a study post or wall stud. Do not just mount the bag on a sheetrock wall. the force of repeated impacts will surely damage the sheetrock.
Rice also works as a filler, but over time the grains will break down and turn to powder. Iv'e also hear of people filling the bags full of copper bb's or lead shot.