There's something that I want to do in my "Not Half Bad" story, but my knowledge of series that have gun-using characters in a modern-day Tokyo setting is rather sparse. This is to avoid getting the police and the military involved. For one, neither appear in Ranma 1/2, even though an event such as Taro and Rouge fighting each other over the city would have had them appear in a more normal setting (even though they made it on the news when they fought in a remote region in China). For another, my dislike for the military in general makes me very ignorant of the ins and outs of it.
At the moment, Cultural Catgirl Nuku Nuku is the only option that I've been able to think of by myself. Of the few other series that I know of, You're Under Arrest involves police in another ward and Gunsmith Cats is based in America. Two other series that have come to mind, that I have only seen a few episodes of (and thus I don't know their feasibility), are Noir and Full Metal Panic.