A straw sale already involves a 10 year jail sentence with a minimum of $250,000 in fines. Can't get much harsher than that.
And I couldn't tell you how we keep guns out of criminals hands, nothing I have heard proposed could do it and nothing I can think of could do it. You can't stop people from doing something by enacting laws, you just cannot keep track of even a small percentage of the things it effects.