The 2nd amendment guarantees the right to bear arms for a militia to defend the country, NOT for free personal use. The NRA is attempting to undermine the strength and stability of the US by promoting gun legislation that is harmful to the American people and contrary to even the broadest interpretation of the Constitution.
Extremist gun activism is promoting the instability of the US gov't, and their member often state that they will kill those who would take them. Since law enforcement is the only people, who could take people's guns, then the NRA are actively supporting their right-wing extremists willing to kill American law officers. This is, based upon Homeland Security's definition, a terrorist act.