Actually from a distance the NRA decal looks like the Marine Corps decal.
On one hand, it is like the admonition to not have your gun cabinet, gun rack or gunsafe visible through a window from the street, or a gun rack, gun cabinet or gun safe visible to casual visitors inside the house. Don't advertise to burglars (although it may deter home invaders).
On the other, avoiding angering antis who might get paranoid or try to "cut you off" in traffic, I read a rant by a lefty about pursuing someone in traffic because they had a Bush-Cheney sticker on their car in 2004. Geez (even some of lefties on that board thought it was excessive behaviour). Looks like (some) lefties (or maybe yellow dog Democrats) still engage in that sort of behaviour: She gets dirty looks and gets cut off in traffic occasionaly. I see a car with a lot of anti-Bush, fish with legs, other left wing slogans, logos and bumper stickers, I just think how tacky and drive on. The idea of dissing someone in public or cutting them off in traffic is just insane behaviour. (Sorry about the -ours I've been reading Dickens in the original English.)