Here's an article from Yahoo News
547 dead, 476 hurt in mass shootings since 1983

By Olivier Knox, Yahoo! News | The Ticket – Thu, Mar 21, 2013

Public mass shootings like the slaughter of schoolchildren and staff at Sandy Hook Elementary have left 547 people dead and 476 more injured in the U.S. since 1983, according to a new report from the Congressional Research Service
Read the full article at:
http://news.yahoo.com/blogs/ticket/5...ccmt-container

And for those inclined here's the complete Congressional Research Service report:
http://www.fas.org/sgp/crs/misc/R43004.pdf

For what it's worth I really like using sources like this against the antis and especially politacal antis because they cannot claim bias on the part of the research.