Okay, what others have told you is the official story.
Now here's the "unofficial" but "real life" facts.
If you bring a firearm into DC, and you get caught with it, they will do everything in their power to make your life a living hell and extract as much money from you as they possibly can along with pressing criminal charges.
It's just that simple.
Until we see a few more court cases and until DC -really- gets slapped down hard a few more times, do NOT bring firearms into DC.