Just tax the bullets to 10 bucks each - make (loading) the gun so freaking expensive.
The problem is that making it expensive to load a gun also means that it's expensive to practice with a firearm too. I think someone with a gun and no idea how to use it is far more dangerous than someone who has taken a safety class and practices regularly. Practice and recertification is a requirement for ALL pilots, maybe something that should be mandated for firearms owners (and car drivers!).
Yeah.. we already debunked the "make bullets expensive" line of reasoning a couple of weeks ago. That's not a viable answer.