This comes up so often we've got a permanent thread (much of which has been 'borrowed' from Researcher and PCGA forum) over on 16ga.com:
http://www.16ga.com/forum/viewtopic.php?t=1694
http://www.16ga.com/forum/viewtopic.php?t=1751
AND the Low Pressure Reloading group over there has all the infro you'd want.