That certainly is a valid temporary solution, Twin. Ive considered doing that myself! But even with a bunch of lead shot I don't think you'll get the heft you want, unless you melted the lead and poured it directly into the cavity. If you got lucky and it didn't catch fire... it might work. lol
From what I gather 14oz to 20oz Poly headed mauls are preferred, for all around use, with 5-9oz being used for lighter more delicate stamping. Balance seems to be important depending on the users method of holding the maul. I tend to old my mallet like I would a 16oz framing hammer... all the way down on the handle. Especially when I am using brute force to get some of these larger punches to go through.
According to the young man at Barry King (not Barry, and I've forgotten his name), the white material is the most resilient, and there was some side note about nylon I don't recall. The brown and black apparently have a slightly, almost indiscernible, difference in feel from the white and yellow.