Re: FBI can't unlock smartphone
One: Well the amendment specifically says "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed" which opens up all manner of possibilities/legal wrangling even while leaving that in place. Moves to extend that right still further are actually quite recent. Also the constitution can be amended if the political will exists, the threats to persons,state and property when this document was written are different to those today.
Two: In phases with amnesties much as other nations have done in the past, you start with the most ridiculous and work your way down. There are also other options to outright bans, a legal requirement to hold and use certain firearms types in designated/certified ranges for example.