It's not "Big Guns" it's over 4 Million members, plus a whole bunch more who follow what the NRA has to say, even if they aren't dues paying members. Most of 'em vote too. That's what gets the attention of the CongressCritters.
BTW, how many dues paying members, or any sort of members, does the Brady Bunch have?
http://www.bradycampaign.org/xshare/pdf/annual-reports/2007.pdf
The window froze when I tried to open it. It looked like it was opening correctly. I tried to use the edit function. IIRC, it is 20 pages long. Their About Us page didn't mention donors or members.