It makes sense, they became political when guns came under political threat.

As a side note, it's funny to see that parts of the media still mechanically repeat LaPierre calling the ATF "jack booted government thugs" in 1995 like it was a bad thing, after the full-on liberal awakening over police militarization via the Boston Marathon Bombing and Ferguson, MO. The NRA was 20 years ahead of the curve on that.

Didn't see the Frontline episode, but the NRA's history is quite interesting. The organization was quasi-governmental in that it administered the government's civilian marksmanship program (this was a war preparation thing) and it ran shooting tournaments and did other educational programs. It's interesting to read about the gun control debates of the 60s (leading to the passage of the Gun Control Act of 1968) because people at the time thought that the debates were very heated but, compared to today, the NRA's position was pretty moderate. The organization was supportive of certain elements of the '68 act, including banning mail-order guns (one of which killed JFK) and Saturday Evening Specials.

After '68 there was a huge split in the organization between hardline types who wanted to focus on politics and lobbying and traditionalists who wanted to focus on shooting sports. At one point the NRA was even going to move from the DC area to Colorado and essentially give up politics altogether. In 1977, the political types took over the organization and purged the organization of sportsmen types.

Interestingly, the new head of the NRA was a former chief of the US Border Patrol who had shot and killed someone when he was a teenager. https://en.wikipedia.org/wiki/Harlon_Carter

The organization grew exponentially in membership, budget and political influence. Interestingly, they went through a few cycles of purging people who were even more hardline than their leadership.