Many Americans, especially those in rural areas, tend to oppose gun control. Even if the NRA did not exist legislators would be in trouble with their constituents if they voted for strict regulations or even advocated it.
Most Americans do not even know their legislators and certainly have no clue on how they voted on any issue. Yet, they are convinced they are taking bribes to vote a certain way. But, nobody ever claims they voted based on who got the most campaign contributions.
Does high-tech and labor unions own the Democratic Party? It has the same level of validity as saying the NRA owns the Republicans. Voters are free to defeat any legislators who do not represent their interest regardless of how much money they spent.
Any blame should be placed on American voters--not any interest group or campaign contributions. The AARP is probably the most powerful interest group in the country and gives no contributions.