501(c)(3) exemptions apply to corporations, and any community chest, fund, cooperating association or foundation, organized and operated exclusively for religious, charitable, scientific, testing for public safety, literary, or educational purposes, to foster national or international amateur sports competition, to promote the arts, or for the prevention of cruelty to children or animals. There are also supporting organizations which are often referred to in shorthand form as "Friends of" organizations.
The NRA is an overly powerful lobbying organization and does not fall into these guidelines in any way, shape or form and should not be exempt of taxes.