I have posted about Heavy Sandbags before: Strongman, sandbags and rehab Now I think rubber mulch is the best filler material. I got it at a landscape products store for $8/25lb. The density and volume is somewhere between pea gravel and wood pellets. My 200lb rubber mulch bag is a beast. I have never had a leakage or burst with a good demolition bag and electric tape even with a 220lb pea gravel bag loaded, shouldered, and weight over bar at sternum height. I dropped a 160lb from overhead multiple times with no issues. Two bags I washed and dried on hot a few times to shrink them and it made them to a bit more sensibly sized.