Like the semi-auto/revolver comparison picture. Mind if I suggest a new version?
Divide the picture into two. In one pane (headed 'illegal in D.C.'), a fairly non-threatening-looking .22 semi-auto pistol, in the other (headed 'illegal in D.C.') a revolver in .44Mag, .500SW etc. Under each gun is a round of its ammunition laid vertical so the size differences are apparent. At the bottom is a short banner caption along driving home how ridiculous the law is, but without implying revolvers or certain calibres should be illegal.