It will be a long road, but every gun turned in and every gun that is not purchased will leave the diehards standing with less company. Every gun owner that divests will make the guy that is amassing an arsenal and tactical gear look noticeably weird. If most people do not buy 1,000 rounds of ammo it will be strange and noteworthy to do so.
The above will be more effective if the purchases are logged. No Supreme Court decision has said that rights cannot be regulated, so tracking sales is constitutional under Heller, and it is only legislative politics in the way.
Not mentioned in most 2nd amendment case discussion is ammunition, although there have been some struggles over allowable types. Once again, I think simply tracking would point to unusual patterns.
I feel we should go straight at gunpowder itself. It is a choke point not easily avoided. Even outlaws with guns need ammo, and loading your own is not possible without smokeless powder, which no one makes at home. Good luck trying to fire an AR-15 with black powder in the shells. Black powder is notorious for absorbing water ("Keep your powder dry!") but even smokeless-powder ammo has limited shelf life.
Perhaps simply prohibiting reloading of shells would drive business to trackable store purchases. Limiting the sale of powder would put teeth in the prohibition, which might otherwise be avoided because shells are in the main re-usable, and lead can be melted and cast. Powder has to be purchased from licensed sources.
The San Bernardino shooters had over 6,000 rounds of ammo, 1,600 with them in the car, 4,500 at home. A few thousand dollars' worth there, depending on the mix. No one needs this, and even 10,000 rounds won't defeat the police when they show up in force.
With the right ammunition limits, perhaps we could defang the gun threat. The police are correct to be worried about guns in car they pull over, or the coat pocket of a suspicious guy in the alley. But if there aren't any new black-market bullets (or too expensive/exclusive), the guy's gun is only for scaring the store owner.
We should not be utopian and think we can shift to a restrictive system where a person has to convince a regulatory body he or she is qualified to own a weapon. We can limit the arsenal-encouraging system and shift to a responsible set of laws, since "responsible" gun owners have failed to protect their hallowed hobby from the scary folks.
I still want gun owners to divest, at least in quantity, to leave standing only the weirdos.