How does Biblical law deal with ownership of lethal weapons (such as guns or weapons of mass destruction)?
Biblical law supports self-defense, including the use of deadly force (Exod. 22:2-3). Biblical law also supports the defense of (innocent) others, including the use of deadly force if necessary (proportionate to the threat).
There is no allowance in Biblical law for civil government to regulate the ownership of weapons, or to prevent a legally-innocent person from buying weapons for self-defense or defense of others.
What about weapons of mass destruction, or those which cause indiscriminate harm?
Self-defense and defense of others must be proportional to the threat. An action of self-defense which intentionally or negligently harms innocent others is not morally justified. Anyone who creates or acquires a weapon that could cause indiscriminate harm -- as opposed to one in which the harm could be focused upon actual impending threats to oneself or others -- could be rightly viewed as someone who is threatening impending, unjust harm to others. In particular, if the weapon were something like a chemical or biological agent, there would be legitimate questions about the safety precautions and the rationale behind how such a weapon could be used justly in a warfare scenario. A weapon of mass destruction would immediately be understood as a disproportional threat to innocent others.
Biblical law allows any of us (not just "civil government") to judge the intent of someone with a weapon, and act accordingly. A person developing or acquiring such an indiscriminate weapon could easily, and rightly, become a legitimate target of self-defensive action on the part of any private citizens who might be threatened by such a weapon. This is a threat to which any person (not just a civil government representative) could react with preventive force (including deadly force, if necessary). This is the same Biblical authority which allows any man to participate in defensive warfare, and be morally justified in taking life.
The Biblical right to self-defense does not require one to have experienced harm before one takes action against the threat. Pre-emptive action against an unknown or imminent threat is allowed by Biblical law, and the level of force would be proportional to the threat or level of resistance.
If I find out that my neighbor is building a nuclear weapon next door, I'm going to get some other neighbors together and go talk to him. Maybe we'll be able to convince him that this is a threat to us. If not, we might have to take other steps. Civil government does not need to be invoked.
What are the risks of pre-emptive action?
If you take pre-emptive action against someone without, for example, being put in immediate imminent threat of harm, or do so with disproportionate force, or without having pursued reasonable, peaceful alternatives (Rom. 14:19, Heb. 12:14), then that is going to be witnessed as injury to another (which is a crime) rather than legitimate self-defense.
