When you can modify a legally purchased gun with ease to become an illegal weapon or a legal effective replacement then you have a problem.
And you see the problem with single state implementation (even though Connecticut's own permit laws reduced gun violence by 40%). The effect of the laws introduced after Sandy Hook (magazine size etc.) are still being measured.
Lack of certainty isn't an excuse for doing nothing when the problem is so undeniable.
Oh and violent felons can buy guns without any background checks in several parts of the US (and online). Saying someone is banned from doing something without preventing them from doing it is like saying you've put someone in prison without building a wall.
I'm Israel, he's Palestine, its more fun when you pick sides.