Firearms is the Issue Hillary Backers Choose to Sink Bernie

It's been said before that Bernie Sanders is no friend to gun owners, having voted to criminalize private sales of firearms and for a ban on modern sporting rifles in 2013.  But because he voted for the Lawful Commerce in Arms Act, and is not willing to go along with every gun control scheme with no questions asked, his is not pure enough for those on the left who strongly support taking away our rights.
Sanders continues to be hammered on the issue.  Late last week, the Christian Science Monitor had a post on it's web site by Matthew Dickinson, a blogger and professor of political science at Middlebury College, titled "Bernie Sanders Gun Control Record Gives Hillary Clinton an Opening":

And while it certainly does not represent a shift in presidential politics (contrary to this Washington Post article), Hillary Clinton has been sure to include a reference to strengthening gun control laws in her stump speech, albeit without mentioning Bernie by name.

Dickinson refers to Sanders' position on the issue as "ambivalent" several times.  The piece also brought out the fact that it's not just Clinton's supporters who think Sanders is vulnerable on the gun issue.  The same article linked back to a previous post by Dickinson that included a link to an ad paid for by a superpac supporting Martin O'Malley:
There is one Democrat who is much more pro-rights than Sanders, but that candidate (Jim Webb) gets no attention for his position because he is no threat to Clinton.  The fact that Bernie is getting so much attention must be an indication that he at least has taken away the sense of coronation that once surrounded Clinton.