Statisticians have frequently noted the difficulty in making such comparisons. Many European countries are much more homogeneous than the U.S. Also, you mention

Statisticians have frequently noted the difficulty in making such comparisons. Many European countries are much more homogeneous than the U.S. Also, you mentioned the porous borders between states reducing the effectiveness of said gun legislation. We have a fairly porous border with Mexico. Mexico currently has more drugs and people than firearms, but there's no reason we couldn't see a marked increase of arms smuggling into the U.S. in the event of stricter gun laws. In many ways it is more straightforward to manufacture firearms than meth, for example.