How do I get a drop shadow to spread the full width of an element. This is what I have right now but I’m it’s falling short of spreading horizontally across the element.
As you can see at the right and left of the image the shadow is inset I need it to span the entire width.
box-shadow: 0 8px 6px -6px black;

Your negative spread-radius is causing that. Change the -6px to some positive value:
jsFiddle example
According to the MDN: