Let me explain why I draw the line at individual versus crew served weapons. In general, I agree with your proposition for safe storage and responsible use. Now how does one safely store a howitzer and it's ammunition? How does one demonstrate that they can responsibly use the howitzer? Who does one demonstrate these things too?
There is a fine line here. I don't want to get into a situation where one has to demonstrate safe storage and responsible usage for all firearms/weaponry. But I do believe that there is a point where the destructive capability of the weapon makes such a demonstration necessary. I think you might agree since you also draw a line at WMD. I want to avoid the situation where a citizen has to demonstrate safe storage and reasonable useage for all weaponry. I draw the line at individual weapons such as pistols, rifles, and shotguns since I believe one can reasonably assume that an individual can acquire the knowledge for safe storage and usage without government certification.
I'm not saying that the ownership of crew served weapons should be outlawed. I am saying that there might not be a right to own these things. You see, I believe that if there is a right, then the exercise of the right should not be subject to government approval. I can see a logical distinction between the right to individual weapons and a need for approval for crew served weapons. I also see that this might be consistent with an individual rights interpretation of the second amendment.