To be clear, Bernie has an excellent record of doing something to reduce gun violence. But it's also clear that his record on this is not as good as Hillary's.
The NRA's grading of Bernie and Hillary reports this accurately.
Bernie received a D- from the NRA, a grade indicating the NRA's evaluation of Bernie's support for the NRA's pro-gun positions, or their staunch opposition for any stronger limitation on sales and use of guns in the United States than what we have now. As a professor, I know full well that D- is a poor grade, a terrible grade. But it is a grade that is a PASSING GRADE. If a student gets a D- in a course, he or she has passed the course, and receives credit for passing the course. The course is listed on the transcript, and the credits are applied towards graduation.
In other words, Bernie passed muster with the NRA - minimally, the most minimally, but he passed it.
In contrast, Hillary received an F from the NRA. An F is a failing grade. Students who receive an F do not get credit for the course. And if they need the course for graduation, or want to get credit for the course, they need to take the course again. An F is thus obviously a lower grade than a D-, and there is no grade lower than F. There is no F-.
As to why the NRA gave Bernie a D- rather than an F, the facts are out there: Bernie voted five times against the Brady bill, voted to allow guns on Amtrak and in national parks, and indeed at times was graded C- by the NRA (and at other times, F). Here's a complete run-down by Politifact. The gist is that the NRA's grading of Bernie is not incorrect. The NRA is horribly wrong in its ceaseless efforts against gun control, but it is capable of accurately keeping track of votes on legislation to limit gun use.
I can understand, completely, why some people might prefer Bernie to Hillary as President. Perhaps they think cracking down on Wall Street and its abuse is more important than cracking down on the proliferation of guns in this country. And Bernie's D- from the NRA, again, shows that he is excellent in his opposition to the NRA and their efforts to limit gun control. I would vote for Bernie in a heartbeat against any Republican, and the A and A- grades they have received from the NRA.
But because I see guns as the single biggest violent threat to life in this country, I'm supporting Hillary in the Democratic primaries. I want the very best person on this daily lethal issue - or, in the NRA's view, the very worst.