Perhaps if the Second Amendment applies only to the federal government it might simply be prohibited under the Constitution's Commerce Cause from using federal funds to enable or support any state or local actions that infringe the right to keep and bear arms.
So the states might be able to have extremely restrictive laws without any interference by the federal government but the federal government might not be able to supply any funding that furthers the enforcement of those laws. For example, individuals currently do not have the right to keep and bear arms in New York State. New York probably could continue that position but, of course, the federal government might be prohibited from allowing the state or any of its municipalities to do NICS checks, use FBI training or other services, obtain government funding for police equipment that might be used to enforce its restrictive laws, and perhaps even be unable to have its police use federally funded highways to travel on the way to arrest offenders, to incarcerate them in jails or prisons built or maintained with federal funds, or try them in buildings built with federal money.
For example, the federal government has no right to impose speed limits for drivers in any state. And of course the federal government would not even think it had such a right because that would be
wrong. So in 1974 President Nixon and the Congress used the Commerce Clause of the United States Constitution to require a 55 mph speed limit as a condition for any state to receive federal highway funds. For some reason or other, all states immediately imposed a 55 mph speed limit. The federal government did not impose that speed limit. The states did.
In the situation that concerns us here--
Heller v. DC--that kind of action might not originate automatically with the federal government. It might require law suits against the federal government, but that's no big deal. All of the people who don't support the NRA because "it doesn't go far enough" (or whatever their individual reasons might be) will have the opportunity to bring those suits in their own states and cities at their own expense, to demonstrate the purity of their principles, and there's no doubt that they will do it.