Is there any type of license a U.S. citizen can obtain to conceal carry a firearm throughout all the U.S.?

I have a conceal carry license in my state, which also works in a couple of other states, but its limitied to those states and only certain places in those states. Do Police officers get to carry their firearms nationwide, or just in the state where they are licensed to be an officer? I ask because I can obtain a private investigator's license which, is on the low end of police enforcement and would consequently give me the rights to carry my firearm as an officer in the state I reside. But, im sure some people have the ability to conceal carry their firearm throughout the U.S. The question is, who are these people, and how do I obtain legal ability like them?