http://www.politifact.com/truth-o-meter/statements/2015/jul/10/generation-forward-pac/did-bernie-sanders-vote-against-background-checks-/
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According to Sanders' campaign manager Jeff Weaver, Sanders reason for opposing the Brady bill was two-fold. First, he believed implementing a national waiting period was federal overreach. And second, he was doing his job.
"He wasn't opposed to states having (waiting periods) if they wanted to. The Republicans wanted to repeal waiting periods in states that had them, and Bernie voted that down," Weaver said. "He said he would be against waiting periods, and he kept his word to the people of Vermont."
In April 1991, Sanders then-chief of staff Anthony Pollina echoed the idea that Sanders was simply representing the will of his constituents.
"Bernies response is that he doesnt just represent liberals and progressives. He was sent to Washington to present all of Vermont," Pollina said. "Its not inappropriate for a congressman to support a majority position, particularly on something Vermonters have been very clear about."
The Green Mountain State, though left-leaning, has a high gun ownership rate and lax gun control laws (as well as a low homicide rate). That and Sanders own personal views are reflected in his overall voting record, experts told us.
"As a rural state with a large number of hunters and other gun owners, Vermont has been less liberal on guns than on most other issues, historically," explained Bertram Johnson, a professor of political science at Middlebury College in Vermont. "He seems to support more regulation of guns than the U.S. presently has, but he recognizes his constituents preferences so does not make gun control a priority."
NOTE: This is a screencap, blue links do not work but can be accessed at article page.

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"When it comes to guns, hes not Ted Cruz, but he believes federal policy should be less intrusive than Martin OMalley or Hillary Clinton," said Eric Davis, who studies Vermont politics at Middlebury College. "Guns are not an important issue for him, because they dont fit into the class-based framework that Bernie looks at politics through."
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The Brady bill imposed a five-day waiting period for would-be purchasers of handguns. Between 1991 and 1993, Sanders voted against it five times. He did, however, vote for a version of the bill that imposed instant background checks, and against an amendment that repealed state background checks.