Your example is fundamentally flawed. Plutonium on its own does not create the threat of a nuclear bomb -- plutonium is used in the manufacturing of nuclear bombs. The only threat of plutonium would be the levels of radiation that it would produce, as such, one would need to make sure that the plutonium is properly shielded in order to protect the public.

That being said, I do understand the point that you are trying to make, and I do agree with it -- if one looks at things through the perspective of the non-aggression principle, an argument could certainly be made that there exists examples of items whose mere existence is a threat to the safety of others. For example, stockpiling large amounts of fertilizer (e.g. ANFO), improper storage, and handling of dangerous pathogens, nuclear bombs, etc. These examples, by their mere existence, creates a threat to the livelihood those around it, as such, an argument could certainly be made that they should be regulated by law to ensure the safety of the surrounding public; however, in general, I do not see firearms as falling within this category, or, at the very least, it heavily depends on context. I would look at it from the perspective of whether or not the situation at hand constitutes reckless endangerment. For example, say you leave, unattended, a loaded firearm on a public bench. This could be argued to constitute reckless endangerment as the firearm could easily be accidentally discharged by an unassuming passerby -- since an item in the public domain could certainly be expected to be interacted with by a member of the public -- thereby creating a threat to the safety of others -- the individual whom is the owned of that firearm could thus be considered as responsible for endangering others. Another example would be leaving a loaded firearm unattended in a residence with children around. This could be argued as negligence for the safety of the child, and could be legally treated as such. However, if your firearm is in a location that, on its own, creates no immediate threat to the safety of others, and the only way for it to become unsafe if it is taken from that originally safe location by an individual who is not reasonably expected to interact with it -- e.g. theft, and trespass -- why should one be responsible for that outcome?