There is no way I would ever put a check mark next to Clinton's name unless I were voting to put her in prison. I would rather see Trump in office than Clinton. (Hell, I'd rather see Trump in office than most of the republicans who ran, and I'm old enough to remember when Ventura was Governor, so I know what I'm asking for.

A president has a certain amount of political capital to burn, favors to collect on, promises to keep, etc. I think Sanders would be unlikely to burn his capital on gun control - he has other priorities. I think Clinton has a hard on for guns, and would be far worse than Sanders regarding the 2nd.
(And I'll be a single-issue voter as soon as there's a single issue candidate, i.e., one who promises to cast every single vote based solely on how a bill would affect the single issue he or she ran on.)