Yeah, unfortunately there may be a simple explanation for the drop in NRA campaign spending. That graph shows that it nearly doubled between 2014 and 2016, ie to elect Trump.
Supposing that were Russian money, the Russians have now bought the White House and don't need to spend again.
Article below is about comparatively small admitted donations by Russian-linked entities, but also about continuing investigations into donations via shell-companies and other means.
NRA got more money from Russia-linked sources than earlier reported
www.politico.com/story/2018/04/11/nra-russia-money-guns-516804
Campaign records show that the NRA was Trump’s biggest supporter, spending about $30 million on him during the campaign, a much larger amount than it has spent on previous presidential candidates.