Understanding NRA-Grading For Lugers ?

Can anyone steer me in the right direction to books and/or web-sites that might give a beginner an understanding of the NRA-grading terminolgy I see in many luger want ads ?


It seems that such grading may be very subjective. And as the proverbial fish gets bigger with each telling of the story, I am beginning to suspect some luger grading (by the seller) may get better with the selling...


Is there a definitive reference book that explains such grading and how such grading is assigned qualitatively/quantitatively ?


Thanks in advance for your help...


Pete