Our gun club is having some disagreements over NRA membership and the definition of firearms themselves?

It is simple, Your answer is, "You knew the rules of this institution when you joined it. Abide by them or go make a new organization with rules you are willing to abide by. Your choice but quit trying to change my club. Period Dot En d of Story." Then call a meeting and vote out those who do not want to abide by the rules.