We're expressly talking of challenges to changing US constitutional law only in this instance, not the NRA opinion of it. Bit of a brief legal history sidebar here...
When the Constitution was signed, federalists claimed the new government would only have limited powers expressly delegated to it. This wasn’t enough for anti-federalists like George Mason, who wanted explicit guarantees to prevent any potential encroachment by federal govt.
Mason wrote:
"A well-regulated militia, composed of the body of the people, trained to arms, is the proper, natural, and safe defence of a free State"
The Founding Fathers, having just broken away from Britain, urealised with unusual far-sightedness that the federal govt they were ratifying might one day in the future become just as tyrannical. If it had the overweening authority to control citizen access to firearms, then it could disarm them, just as the British tried to, and any govt could have a free hand. The 2nd Amendment was specifically included to prevent this.
Over two centuries later, there is an ideological struggle with both gun freedom and gun control advocates. Some ignore or are more likely unaware what the ratifiers had to say, because all germane historical documents contradict them.
James Madison said in 1789 that "A well regulated militia, composed of the body of the people, trained to arms, is the best and most natural defense of a free country."
As to the word "militia", that's some kind of army or law enforcement or something? No, for example it has nothing to do with the National Guard. There is already a legal clause that specifically authorises arming them. Nor is it the US Armed forces, Police forces, FBI, CIA, NSA or any federal agency.
Mason said they were
"... the whole people, except for a few public officials."
Alexander Hamilton wrote,
"if the representatives of the people betray their constituents, there is then no recourse left but in the exertion of that original right of self-defense," a right which he declared "paramount." (Federalist 28).
And then there is clause "... shall not be infringed." There is no exception to this contained anywhere in the amendment.