It has been said that the NRA only looks at the candidate's previous history on firearms legislation.
Not at their parties general position on firearms, nor their policies on any other topic.
They also often times endorse the person they see as being "most-electable" at large, which is often the incumbent.
For example, MN US House of Reps, District 1.
Demmer (R) vs Walz (D)
Both are Rated A (Solidly pro-gun candidate. A candidate who has supported NRA positions on key votes in elective office or a candidate with a demonstrated record of support on Second Amendment issues.)
NRA endorses Walz as he is the incumbent. Walz is a true A on firearms, but is also a liberal on many issues. He is supported by the NRA, but also SEIU, UAW, AFL-CIO, AFSCME, Teamsters, EduMinn. He believes in green energy (like T-paw and McCain) and supports 'jobs programs.'