Guns are such a marginal issue on the left. This is the equivalent of Scott Walker making his grand appeal to the presidency on union busting.
There's a reason despite a relatively consistent majority of voters supporting enhanced gun control it never passes, enthusiasm gap. There's not enough real grassroots support for gun control to battle the apathy that the majority of voters feel towards it.
And I fail to see the big gap between sanders and the democratic mainstream on guns. He wants and has voted for the only gun control measures that have any hope of ever passing even a democrat controlled congress. Where's the space to the "left" of increased background checks, closing the gun-show loophole, and a reinstated assault weapons ban, but short of physically taking guns away?