Show me the left leaning, or even so called centrist speaker willing to speak publically in favor of gun rights, beyond the meaningless platitudes "I support hunting" or "I support the 2nd Amendment".
Go ahead, find one....
If they are out there, we don't see 'em.
I know some folks who are left / liberal / progressive in some of their social views and would be considered hard core conservatives about gun rights.
SO, I know they exist. And I know that they have learned to (publically) keep their mouths firmly shut about their belief in gun rights. Because, if they don't, they get attacked, and attacked personally, for their being "traitors" to the "cause", by the rest of the group(s) that hold the same, or similar views, except for the gun rights issue.
It is a circle that feeds on itself. The NRA is seen as right wing/conservative because their speakers generally are, and their speakers generally are because no left wing/progressive that supports gun rights dares come forward in public.
Anyone who sends in their application and pays the dues is an NRA member. There is NO political litmus test. Even a hard core anti-gunner can be a member. True, they won't find many friends, but they can be a member. The organization is open to all. And all who join do so by choice.