As you said, only 15 Democrats voted against the Assault Weapons Ban. Obama stated that there was a gun problem and it needed to be dealt with; Trump denies any gun problem. It's not facetious at all; it's available in the record. There was nothing that Obama could do. Nominally, as the GOP has control of both houses, they should be able to push through some sort of reasonable gun control.
As far as the Constitution being changed, it has already been demonstrated that the Second Amendment can be reinterpreted with the assistance of bought-and-paid-for Supreme Court judges, bought-and-paid-for senators and congressmen, and the bought-and-paid-for constitutional "scholars" to give the judges an excuse for the reinterpretation of the Second Amendment after more than 200 years of a different interpretation. 

It is you who is missing the point.