As others have pointed out, there are other 2A advocacy groups that are single issue focused. I have a hard time believing that the NRA has accidentally become involved in mass incarceration.

That being said, if the dilemma is gun violence, you've presented a false dichotomy for options. Harsher criminalization likely doesn't do anything, it may just make recidivism worse and temporarily relocate violence into prisons. Economic violence and inequality is the root cause for much of the gun violence in the US (along with some cultural psychological heritage related to frontier life and individualism, but this is a more difficult issue to solve). Deep poverty, racist and invasive police, lack of access to nutritious food, lack of access to childcare and quality education, etc. Not just for people of color (although they disproportionately suffer from inequality), but for white poor people as well. Economic violence is the backdrop for violent crime, and if the NRA actually cared about solving their image of being an advocacy group for middle income whites and cops, they would spend time lobbying to solve these issues, not locking up poor people.