Yup.

EDIT: I see someone else commented that they're not the same. The truth is... it depends.

When I was serving in the military, we used the two terms interchangeably. They were basically the same thing to us. Although "military grade" is a more common term in the civilian sector, we would still use it when working with contractors on custom military equipment.

So if you want to be pendantic, mil-spec may be the more official term in the military. But in practice, we don't really differentiate between the two terms.