As one who has been on the political scene for over 60 and have assisted many candidates, I can tell you the NRA process is far from perfect. The NRA relies heavily on responses to a questionairre sent to candidates. Anybody can lie with their answers to get a good rating. Voting records are used also but they seem to not weigh as heavily on the NRA position as the questionairre.
Many 'anti' Democrats have been endorsed simply because they said what the NRA wanted to hear.
As far as your "challenge", that could not have been difficult. The endorsements are published in the AR magazine and are on-line.