There use to be (maybe still is) a small faction of the Democratic party that is pro-gun, calling themselves something like "Democrats for the Second Amendment".
In the House there are 14 incumbent Democrats rated B or better by GOA. There are no favorable ratings in the Senate. You can't really rate Democratic Senators per se, because they always vote by party, the minority always in opposition. What you can do is grade their speeches in debate and whether or not they had something to say. Quotes and written statements would be another way, but that is hard to track. The real problem is that if pro-gun, why would one be a Democrat, if the culture was to not be independent in voting.
I don't know that many or any run on gun issues or claim credit for votes on gun control, so it is real debatable whether they represent anyone but themselves when they vote on gun issues. We would do well to force them to address gun issues during their campaigns and hammer them when failing to uphold the Constitution. That helps the electorate see that opinions on gun control should not be a matter of free rationale but are constrained by the Constitution, namely the BoR and the 2nd and 14th Amendments. It is the same rationale we use in convincing members of Congress how to view gun control legislation.
I am not sure I want to have a showdown on repeal of the 2nd Amendment, but that is the only proper gun control legislation. If anyone wants to try that, I wish them luck on getting the attention of 50 million gun owners who can and do vote. Some, possibly in great numbers, would respond in even more patriotic ways. Basically, no one has the stones to try that, and that's the way we want it.
You can rightfully hammer any Democratic candidate, for the Senate at least, for being anti-gun, because no matter what they say, you know how they will ultimately vote. There is no point in even drawing them into debate on the issue. The only thing that would make a difference is if a Senator was determined to be independent and seriously confronted his or her constitutional oath of office. Otherwise, Democrats, at least Senators, are anti-gun by definition.