What stops them from buying an illegal AR-15?
Why a kitchen knife and not a truck? Why a kitchen knife and not a bomb? Why a kitchen knife and not a flame thrower? There are so many ways to kill people, but you pick the one that would cause the least damage... Why? This just makes it look like you either have no imagination or are intentionally trying to make guns seem worse than anything by comparing it to something weak.
Posts like this make it really hard to take people like you seriously. You have NOT thought through what you're proposing.
You don't know anything about the black market.
You think metal detectors are unrealistic when they've been used for a very long time and have stopped thousands of kids from entering school with a gun.
You probably don't even know what open carry laws are or what states even allow it.
You probably legitimately think taking away guns is a good idea and will save lives, but you haven't thought it through... Who's going to take the guns away? How are they going to take the guns? What happens when people resist?