An awful lot of these guns used in mass shootings are guns legally purchased. Maybe part of that "well regulated" aspect of the 2nd Amendment should be that the government is entitled to know where every one of them is at all times. To that end, we could just put a tape-on RFID chip on every gun of every type. That way venues (like hotels) could have RFID readers that automatically detect and track weapons brought through their doors and elevators. RFID chips cost pennies and the readers are just a few dollars. We could make it a felony like filing off serial numbers to possess a gun without an id tag. If we made it law, everybody but criminals would get tags, but that is great because it turns unregistered weapon possession charges into a prison offense. Criminals would begin to think carrying an untagged gun is just an automatic prison term.

Then, a hotel could detect when somebody is bringing guns in and ask to see their concealed carry permit, or call SWAT if somebody brings a number of guns in.

This is one thing that actually has a technological fix.