The orthodox position within the Democratic Party is to demand gun control measures on a national level, even though extensive recent experience tells us that they cannot pass. Sanders, partially as a reflection of the views of his Vermont constituents, and partially for other pragmatic reasons, believes gun control should be addressed on a state and local level. As one would expect, his departure for orthodoxy cost him in the first debate.
In my opinion, that is unfortunate. It is undoubtedly true that gun control measures work better on a national level, but, practically speaking, that is spitting in the wind, so legislation on a local level is the best we can do. In addition, whether we like it or not, guns are an important part of rural culture; the Democrats would be wise to acknowledge and defer to that if they ever want to be competitive in rural areas.