This pretty much sums it up:
“18 U.S. Code § 926A - Interstate transportation of firearms”
http://www.law.cornell.edu/uscode/text/18/926A
Yes: but in this specific case, on every stretch of my 7 hour journey EXCEPT the five minutes in Maryland I am legally permitted to carry a loaded weapon on my person. Not “unloaded, locked up, and inaccessible to me”. That’s the part that I find absolute garbage.