I have a sticker of a pug on my back glass. It has a little speech bubble that says "Snort". I get lots of compliments on that one. I don't normally lock my pug up in my glovebox though so even if they did break in, the chances of them stealing my snorting, farting, flat-faced comrade are slim to none.
But on the gun sticker thing... Good call. I once put a Glock sticker on my car and took it off about a week later. Cars are not the place for political or firearms-related stickers. You just never know when you're going to cross some nut job that sees politics differently than you and he turns around and keys your car or something.
Funny story... A few years ago in college I made a sticker that said, "George W. Bush for Emperor". I had about 1000 of them printed. It was a joke because there was talk of GW being in cahoots with the Chinese military or something (I don't even remember). Anyway, I slapped them on telephone poles, street signs, bar booths, tables, chairs, other people's cars, lockers, bathrooms stalls, you name it, I put it there. And I put one on my car. I walked out one morning to see my sticker gone off my car. I put another one on there because I had a gazillion of them. A few weeks later, I came out and saw that someone had written "F**k You" on the sticker in permanent marker. Okay... Fair enough. I replaced it. That sticker was removed a couple more times. But not as many times as I had people mention it. Love him or hate him, the sticker worked both ways.
I guess I'm trying to say that people DO notice stickers. You might think, "Eh, it's just a Glock sticker," but to someone else, it's advertisement for the gun that took their brother's life. Or the gun dad was carrying when he was gunned down on duty. Whatever. It has the potential to attract negative attention to you and your personal property. Definitely a good idea to not put them on your cars.
Another funny story related to the GW sticker... I was in a Piggly Wiggly parking lot one day and a man in a very large 4x4 said in a very southern, almost comical accent, "Hey man! I like that sticker! It's the damn truth! I mean, think about it. If George W. Bush was emperor of China, then everything made in China would actually be made in America. Hell yeah! Good thinkin' on that one bud!"
Logic, people. Logic.