As Americans reel from mass shootings, law enforcement officials and experts on extremism are taking increasing notice of the sprawling online space devoted to guns and gun rights. That includes gun forums, tactical training videos, websites that sell unregistered gun kits and social media platforms where far-right gun owners swap practical tips and talk of dark plots to take their weapons. It’s an ecosystem rich with potential recruits for extremist groups exploiting the often blurry line separating traditional support for a Constitutional right from militant anti-government movements that embrace racism and violence.