If 'assault weapon' ownership is banned, who is going to come and take them?

Via the Daily Camera: 

The Boulder City Council voted unanimously Tuesday night to advance a ban on the sale and possession of assault weapons, bump stocks and high-capacity magazines in the city...

It will become effective as soon as it's adopted. At that point, according to rules the council has agreed on, citizens who own bump stocks will have to get rid of them within 30 days of adoption. They'd have to get rid of magazines with the capacity to hold 10 or more rounds by Dec. 31...

Assault-style weapons, under a specific and technical definition Carr has written, could not be sold or possessed under this law.

Read more here.