Well, they are way too liberal with their "A" rating... I can't see how ANY democrat can have an A in today's political climate. The democratic party as a whole takes so many donations from anti gun organizations that simply being associated with the party should get you a C. Someone should only have an A if they have drafted or voted for pro-gun legislation or attempted to repeal anti-gun legislation.
Grades should only be given out based on that last sentence. Your percentage (A-F) should be based on your votes for pro-gun or repealing of anti-gun over votes on all gun-based legislation. Would be easy to calculate for large organizations like the NRA that keeps tabs on these things.