Here's an idea: A new bi-partisan federal law making the use of any military grade weapon in the commission of any crime an "Attack on our Homeland' and an 'Act of War.'
Therefore the crime would fall under the purview of the military and prosecuted by a tribunal.
It would allow for a relaxation of normal civil liberties where terrorists and theiir cohorts often exploit our liberal restrictions on investigations and prosecutions. It would not remove a single gun from anybody, but it would change the implications of using an assault weapon, a rocket launcher or any war weapon in the commission of any crime. And when an AR-15 is used in a massacre such as Orlando, the new law allow our military to get involved, as they should be.
Would such an act be possible? Only if all Americans decide we are at war, and when it is happening in our own backyard we need to respond appropriately.