I understand you've suffered some reverses with your regime of gun bans, confiscations and prosecutions. Palmer v. D.C., while unlikely to uproot a fundamentally flawed systems of elitist beliefs, must certainly give pause to any reasonable person committed to the rule of law.
For good or ill, however, you have been granted a 90 day grace period to develop a set of gun laws for your jurisdiction that are, in fact, actually constitutional. This is not a disaster, rather, it is an opportunity.
Look around at what has worked elsewhere in the country. Look to Seattle, a city of roughly the same population, and to Portland, OR (with somewhat lower population) and even to Salt Lake City - and look specifically at their per capita rates of violent crime (it is, after all, the number of injuries and deaths caused by unlawful violence that counts - not the mechanism by which those injuries and deaths are caused, barring a clear correlative relationship).
I would offer, out of good will, the following as notions for a proposed ordinance:
1: Recognizing that firearms on federal property are already thoroughly regulated by the federal government, the D.C. city leadership need take no action beyond passing what is essentially a revenue measure, duplicating federal statute.
"It shall be unlawful for any person to have in their possession any weapon on or in any federally controlled property where federal law bars such possession. Persons violating this ordinance shall be guilty of a misdemeanor punishable by not more than 364 days in custody and a fine not to exceed two thousand and five hundred dollars. This ordinance shall not apply to any person covered by the federal LEOSA statute, a member of the United States Military on active duty, any person honorably discharged from the U.S. Military nor any person holding a valid license from the political subdivision in which they are resident permitting them to lawfully carry a firearm."
2. Recognizing that "gun free zones" are, at best, historically counter-productive it should also be recognized that declared "gun-free zones" are best avoided.
3. Recognizing under both evolving federal constitutional jurisprudence indicating that lawful carry of arms is a fundamental constitutional right and that further, in jurisdictions that while previously forbidding such that upon legalization of lawful carry of arms that blood has not run in the streets, that crime has not skyrocketed and that the sky has not fallen.
Further, it not having been shown that living outside of the District makes persons uniquely evil or incompetent, and further that it is a reasonable analysis that issuing authorities in other jurisdictions are both competent and cognizant of public safety, it is reasonable to recognize the permits of such other jurisdictions and their political sub-divisions. Thus, the following carry ordinance is suggested primarily as a revenue generator:
"The Washington, D.C. Chief of Police shall issue to any citizen of the United States or a lawfully present alien therein not otherwise disqualified by federal law a permit to carry readily concealable small arms either openly or concealed about their persons in all places within the District not specifically forbidden by specific District ordinance or Federal statute upon payment to the Chief of Police of the sum of $150 and the successful completion of a background check or the passage of ten days from the initial application to the chief. The Chief shall have the authority to, in circumstances of unique risk to the applicant or their immediate family to grant an emergency permit to carry readily concealable small arms.
Further, similar permits issued by any of the United States, its territories or the political subdivisions of the States or Territories shall be recognized as being entirely valid within the District while the holders of such permits are either in transit through the District, visiting the District for periods of less than thirty days or while temporarily resident in the District for up to sixty days. Such permits shall also be valid for any federal elected official or their immediate staff who maintain a primary residence outside of the District."
4. Given that every known form of wrongful violence performable by humans with or without the use of small arms of any sort is already unlawful under numerous statutes, no further commitment of time or effort is required in this arena.
The above measures and policy recommendations should serve the District well. Short of issuing hand grenades to the felonious population of the District, it is difficult what policy measures could do much worse than the status quo ante. Certainly, recognizing the right of law-abiding citizens to defend themselves from the criminal and the crazed as said citizens go about their lawful business seems unlikely to cause any great harm - other than to the criminals and the crazed seeking to prey upon them.