Why would you not just leave the firearm at your home? It doesn't seem safe or smart to bring it in your car to DC just to leave it in a parking garage, and it's very easy to get in trouble while trying to fly with one.

If you must leave it in your car for some reason, buy a gun safe and secure it in a locked trunk. If you drive some sort of hatchback or SUV where you can't conceal it in a trunk, don't do it. The last thing you need is for that gun to get stolen from your car and used in a crime.