After doing research, I can tell you that the NRA has the most extensive summary of the laws at http://www.nraila.org/news-issues/issues/workers-protection-private-property.aspx. You’ll find the opposite side of the coin at http://www.csgv.org/issues-and-campaigns/guns-democracy-and-freedom/guns-in-the-workplace.
For a comprehensive media report on this issue, go to http://www.bloomberg.com/news/2012-12-12/guns-to-work-laws-spread-in-u-s-as-business-fights-group.html. You’ll find a great overall report from PLI at http://www.bakerdonelson.com/files/Uploads/Documents/Guns_at_the_Workplace_5-520-4933.pdf.
Finally, check your state law summary on HR That Works.