We have occasionally talked about self-defence weapons, with quite a few disagreements on all aspects of the subject. Part of the problem, to my mind, is that a perfect self-defence weapon has a bunch of conflicting requirements: compactness, ease of use, availability, stopping power (lethal or nonlethal) and reliability. Added to these conflicting requirements, we have a whole gamut of legal issues which limit the solution space which can be realistically explored: limitations on substances, types of weapons, their form and operation and so on.
To really game this one out, I propose that we completely throw out the rulebook and try to imagine the ultimate self-defence weapon. Again, this must be something that anyone can use, that is always available, that always works and that kills or incapacitates instantly.
Have at it gents.