Thats the sh*tty part.
Other countries, which banned weapons outright (there are few) introduced their bans well before there was an accumulation of weapons. Japan was doing it since the 16th century.
What you are asking is for either 1. an ineffective policy or 2. the criminalization of otherwise peaceful people and more excuses for police brutality.
My policy is usually, "police first." If the police disarm first, then I might support putting weapons in collective municipal caches for common use, otherwise why should the common people be disarmed?