DC is a city. What practical use is there of carrying rifles or even handguns to rallies in a city? Because of the nature of cities, the concentration of population and closeness of buildings, it is next to impossible to know where a bullet is going to end up. The gun owner may find himself under a charge of manslaughter or endangerment if the bullet does not end up where expected.

BTW I'm not against guns, I own a few, but I live in the exurbs. Municipalities should have the right to their own laws given their particular situation and issues.