I think if the NRA didn't represent its membership, its membership would not continue to increase, and those members would not continue to part with their money to support the organization.
The NRA's income is over $200,000,000 a year. A very few companies, such as Smith & Wesson, Ruger, and MidwayUsa, can afford to give the NRA $1,000,000 each per year (these are well publicized donations, because they buy much good will with gun owners). There simply aren't enough players in the gun industry as big as those three, who are also willing to give the money regularly, to support an organization that spends over $200,000,000 a year, and there is simply no incentive for large secret donations by gun companies.
As spelled out in the Atlantic: http://www.theatlantic.com/business/archive/2012/12/whom-does-the-nra-really-speak-for/266373/
Between  and 2011, the Violence Policy Center estimates that the firearms industry donated as much as $38.9 million to the NRA's coffers.
So even the most anti-gun-rights group in the country, only claims that the NRA got $38.9 million from the firearms industry in a 6-year period. That's 3.2% of their budget.
Examine this from a game-theoretical perspective, and ask yourself, "does it make sense to say that the NRA does not represent gun owners?" "Who would be in a position to hurt the NRA the most if the NRA decided to represent manufacturers at the expense of owners?" "If the NRA does not represent gun owners, why does their membership continue to increase?"
When your conclusions don't match the data, maybe it's time to reexamine your assumptions.
I do understand the point you're trying to make about the NRA: you're arguing that because they only support one civil right, they are not a civil rights organization. Would you similarly say that because the American Civil Liberties Union does not speak out for stronger third amendment protections, they are not a civil rights organization? Or that because the Electronic Frontier Foundation does not support ninth and tenth amendment lawsuits, they are not a civil rights organization?
The point I'm making is that you can't plug your ears and say, "they're not a civil rights group," simply because they either support a civil right you don't agree with, or fail to give support to one you do agree with.