This is a hot potato but I will try and address it. I want to focus on the history of the NRA, and what I perceive your question to be: whether the 2nd Amendment is an individual right or a collective right (which is U.S. v. Miller).

After the Civil War (ended 1865), and during the ensuing several decades, Americans (particularly in the Northeast) began forming lots of social clubs, groups, etc. As the number of sports (college club sports, which became intra-college games, and also professional games) increased, there was a need to organize the rules of the game. You had the American Kennel Club established in 1884 to standardize dog breeds and dog competitions. You had the National League founded in 1876 standardizing the rules of baseball. United States Lawn Tennis Association in 1881 (which became the USTA). United States Golf Association in 1894. And you had the National Rifle Association, which was founded around 1871 to both increase the level of American marksmanship and also standardize the rules of competitions for shooting ranges across the country.

Much of its work for the first several decades was similar to the work of these other sporting groups; it standardized the rules of competitions so that an NRA match in New York would have the same course of fire, size of target, and rules as one in Philadelphia of Chicago, and it oversaw or assisted in the administration of local clubs or feeder clubs. Unlike the other sports sanctioning bodies mentioned above, it started off politically well-connected, and at various times was able to obtain money from state legislatures (New York) or Congress to either build shooting ranges or to subsidize shooting teams. So the famous Creedmoor range was built by the NRA, with money from New York State, at 79-26 Winchester Blvd, Queens Village, NY (current site of the Creedmoor Psychiatric Center). It was used for international shooting competitions (against the British) for several decades. These long range matches both placed an emphasis on shooting skill, and also on technical development of rifles and ammunition; something that was seen as socially useful because the US had a small standing military in non-wartime and didn't have a fully developed military-industrial complex.

Prior to the National Firearms Act (which was the subject of US v. Miller), the NRA's lobbying effort was basically directed at getting states or Congress to set up programs to provide free ammo or used guns or in other ways subsidize its shooting clubs, under guise of this will make for better riflemen in the event of another major conflict, draft, World War, etc.

And from 1934 (NFA) through 1968 (Gun Contrl Act of '68), the NRA either backed federal gun legislation or was neutral on it. It's monthly magazines were not political, but mainly had articles about hunting and shooting, reports from competitions, and articles on technical firearms issues (ballistics, chamber pressure, optimal barrel length, how to get the best accuracy, etc.). For instance, this issue of the NRA's leading magazine, American Rifleman, from 1954 includes articles about what to expect when you compete in your first smallbore rifle match, how to go sheep hunting, how to make a maple rifle stock yourself, how to hunt woodchucks (the growing sport of varmint hunting in post-war America as rifles became more accurate), how to introduce kids to formal competitive target shooting, shooting Confederate long guns, understanding telescopic rifle scopes, a recap of the 1874 rifle match between Ireland and the United States, etc. It's like reading Popular Mechanics of that era, but focused on guns. You can read the an archive from 1923 until 2015 here: https://archive.org/details/pub_american-rifleman?&sort=-week&page=3

America changed in the 1960s; with unrest in cities and rising urban crime rates (or at least the perception of such). Some politicians began urging gun control as a political issue, including urban Democrats, urban Republicans, and even Ronald Reagan when he was governor of California. This caused a crisis (or coup, depending on your perspective) within the NRA.

Between 1975 and 1977 there was internal dissention within the NRA, culminating with Harlon Carter (longtime member, former President) becoming the chief lobbyist. He was then fired by the Board of Directors, and then got the bylaws changed and was put back into power as the Executive VP. Carter ran on, and was elected or selected for leadership, because he wanted the organization to pivot from away from merely being neutral voice of setting rules for competitions and into getting politically active lobbying organization (which he had, for years, advocated a no compromises, no legislation stance). It is unclear whether Carter created the "no laws, no compromises" stance but it is one that numerous other lobbying/political groups have adopted and it is a great way to raise money and fund candidates, regardless of the political issue.

Through the 1980s and 90s, the American Rifleman, and other gun magazines, continued to have articles about ballistics and shooting trends, but also contained legislative updates, articles about pending legislation, and encouragement to contact your representatives. Part of this was supporting the then-nascent idea of an individual right to firearms, which was coupled with the growing trend across several states of allowing carrying pistols if someone had a concealed carry permit. There was also a growing whiff of taking a position against certain animal rights and in opposition to anti-hunting groups (like PETA and Sierra Club) which were gaining prominence but were generally in opposition to the issues important to the membership (hunting in general, using lead bullets, etc.).

But all of these gains would be for naught if the 2nd Amendment, as construed under Miller, is a collective right and only relates to each State's right to form and organize a militia, and not an individual right of each person to own a firearm. The NRA (and gun industry magazines, available at every grocery store during the 1990s) began to publish that the 2nd Amendment was an individual right. Even if the NRA's membership were only 2 million people, these 2 million believed that. And they would mobilize and write their congressmen. At the same time, the growing conservative legal academic movement (the Federalist Society, etc.) either adopted or further developed this legal theory. It goes to far to say that the Federal Society was directly funded by the NRA; I don't think they were. But they were traveling in the same ecosystem of donors, Washington DC lobbying circles, etc.

For the last several years, the NRA's leadership has been racked by scandals related to self-dealing, excessive spending, etc. Setting the "how non-profits can go bad" narrative aside, the NRA story is a compelling one of how a hobby or club organization can (1) create a message to inform its members and then (2) create political action on behalf of its members. Usually single issue political groups aren't connected to a particular sport or hobby, such that the sanctioning body can use its platform as arbiter of the hobby to consolidate and mobilize a voting base. Like, you haven't seen the ice fishing industry come out in strong support against greenhouse gases or climate change.

A major part of the NRA's operations are still the boring and benign operation of shooting matches, assisting private ranges in getting insurance, training and certifying range instructors and safety instruction, etc. Another entire section of the organization is youth and scholastic safety training and competitions.

But it's direct lobbying and associated spending from its affiliated groups makes it a large spender on outside expenditures in US politics, and the entire budget is focused on one issue that can be a hot button issue. While it's lobbying effort is paltry compared to the heavy hitters - the defense industry spent $126 million in 2017, the financial/banking sector spend closer to $500 million in lobbying - but the NRA's legislative arm and PAC groups is focused on just a single issue, and it drastically outspends the pro-gun control groups.