OP, I think you have two different and valid concerns:
1. Promoting NRA and opening up the field for discussions
2. Advertising you have a gun, loosing your element of surprise and exposing your vehicle for break in.
I personally appreciate people with stickers on their cars, although I don't have them on mine I choose to park next to cars with NRA stickers whenever I can. The bad guy would probably go for the car with the sticker and hope for a lucky day and a gun left in the vehicle.
Seriously, support whatever one wants, as long as one doesn't mind the confrontations and exposure - to each, his her own.