He does support gun control. But his record isn't 100% pure enough for the most strident anti-gun people.
On top of that, I think his position did evolve over time, which is also fine.
Sanders Votes for Background Checks, Assault Weapons Ban
Wednesday, April 17, 2013
WASHINGTON, April 17 Sen. Bernie Sanders (I-Vt.) today voted for expanded background checks on gun buyers and for a ban on assault weapons but the Senate rejected those central planks of legislation inspired by the shootings of 20 first-grade students and six teachers in Newtown, Conn.
Nobody believes that gun control by itself is going to end the horrors we have seen in Newtown, Conn., Aurora, Colo., Blacksburg, Va., Tucson, Ariz. and other American communities, Sanders said. There is a growing consensus, however, in Vermont and across America that we have got to do as much as we can to end the cold-blooded, mass murders of innocent people. I believe very strongly that we also have got to address the mental health crisis in our country and make certain that help is available for people who may be a danger to themselves and others, Sanders added.
The amendment on expanded background checks needed 60 votes to pass but only 54 senators voted for it. To my mind it makes common sense to keep these weapons out of the hands of people with criminal records or mental health histories, Sanders said.
Under current federal law, background checks are not performed for tens of thousands of sales up to 40 percent of all gun transfers at gun shows or over the Internet. The amendment would have required background checks for all gun sales in commercial settings regardless of whether the seller is a licensed dealer. The compromise proposal would have exempted sales between family, friends, and neighbors.
http://www.sanders.senate.gov/newsroom/press-releases/sanders-votes-for-background-checks-assault-weapons-ban