Here are ZB volumes figures:
Final Futures Volume / Open Interest
For May 14, 2003
Last Updated: May 15, 2003 10:06 AM CST
30 Year U.S. Treasury Bonds Month a/c/e
308,144
http://www.cbot.com/cbot/quotes/futures_volume_detail/0,1589,12+410+415+8,00.html
ZN a/c/e volume
594,007...