As far as what to fill it with... HAHA, I've seen them filled with EVERYTHING... some people have gone down to Lowes and bought the insulation that you have blown in your walls, I've seen others fill it with old clothes. Actually, I've seen that done a lot. Ghetto, I know.
My experience with water filled bags... the bladders break after 2 months. Maybe I'm just that badass, but I'm willing to bet not. Not to mention, they have a real weird bounce when you hit them (hanging water filled bag).
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