Quoted:
Are you talking about fissile devices, or just dirty bombs?
I think the material necessary for either would be pretty much impossible to get, even if the NFA didn't exist.
Depends on what you use to make the bomb "dirty". You would be surprised at how much non weapons grade
radioactive material is floating around America in various industries. Some of it is highly lethal. It won't go "boom"
like plutonium but when mixed nicely with ANFO or some other garden variety boom stuff will make noticeable
areas go tick tick tick on the geiger counter. Tactically not a very usefull device but psychologically devastating to the
populace. Although if used creatively such devices could act as an area denial weapon making strategically important
areas difficult to inhabit or make use of.