AskHistorians

It was a gun education and safety group, primarily involved in running shooting events. It's political involvement really began in 1977 when extremists took over the organization. You can look up "The NRA Revolt at Cincinnati (May 1977)" for more info. From there it became a right wing money funnel, eventually gaining the attention of international interests, most notably Russia.

There are various conspiracy theories that may or may not hold water about it being tied to roll over in the CIA during Jimmy Carter's administration. It's an entire rabbit hole tying in George H. W. Bush, Iran Contra, and the CIA rolling governments in central America. There is most definitely some truth, and a lot of out right fabrication involved.

If I understand correctly it was mainly gun manufacturers that helped boost the NRA and helped fund their lobbying the government. Apparently later on Russia had been found to be funneling money into US politics via the NRA as well. Major groups like this are rarely grassroots.

Edit: I just noticed the community this is so feel free to execute me or whatever the punishment is for just being some uneducated guy not posting sources, maybe the iron maiden or something? lol

They got pretty big as a sanctioning body for shooting sports and local shooting organizations, which came with a lot of affiliations with gun manufacturers. Once it became apparent to those manufacturers that the greatest threat to their business was not a decline in enthusiasm among consumers, but a growing risk of regulation the org pivoted into a primarily political lobbying organization. THEN it became apparent that gun rights were a useful wedge issue and right wingers and foreign governments identified it as a great inroad to connect their agendas to a common issue, and here we are.