As far as the NRA grades, those are very simple - the NRA grades on past performance. The NRA picks certain votes as graded (for example the nominations on Kagan and Sotomayor) and if you vote 100% with the NRA, you get the A+.
Unfortunately, as we all know, past performance is not a guarantee of future results. Rep. John Dingell (D-MI) sat on the NRA Board of Directors. He was the originator of the "ATF are jack-booted thugs" comment. In 1994, he voted against the ban until the party told him to toe the line, then he resigned from the Board and voted for the ban.
Bill Clinton had great NRA grades as Governor of Arkansas. We know how that turned out ultimately.
Most recently, we had Tom Coburn, an NRA A+ rated, GOA A+ rated, Senator trying to propose a universal background check that was even broader than Schumer (F) - Toomey (A) - Manchin (A).
You can blame the NRA for what votes they grade maybe; but you can't very well blame them for failing to guess which of the politicians that have voted with them 90% plus in the past are going to backstab them. As I noted, even GOA had problems with that and groups like SAF were backing a Schumer-sponsored bill! Blaming the NRA for not predicting those grades accurately is holding them to a standard nobody else met.