How do you remove 400 million active firearms from the entirety of the nation? The situation with Australia is not analogous, you cannot map that one solution to a completely different context.
Constituents do not want to have their firearms taken away, the politicians aren't corrupt in this situation, they're doing exactly what they were elected to do. NRA's contributions to politicians aren't that impressive, what's impressive is that the organization has millions of members who very actively vote for their interests.
How do you explain a Breivik that happened in a heavily regulated country like Norway?
What do you want to do about trucks? Should we ban them too?