I'm not sure how this would apply to DC (because they are above the law, of course!), but I don't know if they can completely prohibit you from travelling through with a properly stored firearm. I'll quote the text and a source to verify below:
United States Code, TITLE 18, section 926A get it here http://www.law.cornell.edu/ click on constitution & codes - us codes then title 18, then search for section "926a" - > Sec. 926A.
"Interstate transportation of firearms - > Notwithstanding any other provision of any law or any rule or regulation of a State or any political subdivision thereof, any person who is not otherwise prohibited by this chapter from transporting, shipping, or receiving a firearm shall be entitled to transport a firearm for any lawful purpose from any place where he may lawfully possess and carry such firearm to any other place where he may lawfully possess and carry such firearm if, during such transportation the firearm is unloaded, and neither the firearm nor any ammunition being transported is readily accessible or is directly accessible from the passenger compartment of such transporting vehicle: Provided, That in the case of a vehicle without a compartment separate from the driver's compartment the firearm or ammunition shall be contained in a locked container other than the glove compartment or console."
Now my question is, what does "Notwithstanding any other provision of any law or any rule or regulation of a State or any political subdivision thereof" mean? That any State can write a law that overrides this? I've always assumed that this allowed me to travel legally with a legally stored firearm as long as it was legal in source and destination. I take it that means that's not true in DC.
I also have to ask myself what the people who formed this Country would have said if the Brits told them they couldn't possess or transport firearms. Oh, never mind. I guess we know exactly what they had to say about that. I wonder what we should say/do....
Black Fox