Then do they have a secret / trick to disable all briefcase bombs? The Davy Crockett nuke is under 50lbs and was designed for infantry to use (in a almost certainly suicide manner). And then plenty of other types were invented as time went on.
If you can't get them with the big guns, you get them with the small ones, which are still devastating. There are still layers-upon-layers to ensure MAD, as far as I can tell.
https://en.wikipedia.org/wiki/Atomic_demolition_munition
And let me remind you: the pipebomb guy from Jan6th hasn't been found yet, despite modern surveillance methods and nearly a year of FBI searching and interviews.