Per the FBI's Definitions of Terrorism in the U.S. Code, the NRA is a domestic terrorist organization.
Excerpts:
18 U.S.C. § 2331 defines "domestic terrorism":
"Domestic terrorism" include activities with the following characteristics:
Appear intended (i) to intimidate or coerce a civilian population; (ii) to influence the policy of a government by intimidation or coercion;
18 U.S.C. § 2332b defines the term "federal crime of terrorism" as an offense that:
Is calculated to influence or affect the conduct of government by intimidation or coercion, or to retaliate against government conduct.
The NRA uses its elected official "report card" to retaliate against lawmakers who try to pass gun legislation. Its promotion of open carry and gun proliferation threaten and endanger citizens.