Well I read the article below about using heavy sand in the rear bag so I bought some today. I finally found it at Canfield & Joseph (a foundry supply business). I paid $28.46 for a 50lb bag of chromite. If youre interested in this their number is (800) 364-5452.
http://www.6mmbr.com/sandcomparison.html
http://www.6mmbr.com/sandcomparison.html