Personally, small-arms should be enough to make the King of England think twice.
Or Prime Minister Tony Blair, as we are seeing in Iraq  :-D
Where do we draw the line on what’s legal?
Good question, Mary.
I personally think the legal level should be around or just above what would be readily feasible to manufacture in one’s garage. For example, I am pretty confident that I could make a crude gun in my garage. None of the components necessary would be difficult to obtain or fashion into the shape/form I need. Of course there is always the chance you could kill yourself and others by trying this (which is why I won’t) but if you were desperate, then you might take this chance anyway.
As far as explosives, go back 50 years or so and you will find that you could buy dynamite at the local hardware store (its not too terribly difficult to make explosives in your garage, as Timothy McVeigh did, for that matter) no questions asked. In fact, after my grandmother died and my relatives were cleaning out the house, they found some blasting caps :-o, which my dad disposed. I think we had less of a problem with explosives then than we do today, and there is no question that there were fewer problems with guns - because people were responsible with these items.
And this responsibility on the part of the general public is what is needed, IMHO, to prevent accidents and problems. More laws and restrictions aren’t going to work.