From The National Sentinel:

Remember when Boulder, Colorado banned the ownership of AR-15 rifles? They also banned high-capacity magazines and bump stocks, giving gun owners with such magazines until July 15, 2018 to either sell them or dispose of them—whatever that means. Most likely, you’ll have to turn them over to the authorities. Now, there is a grandfather clause allowing residents who already owned AR-15s to keep them as long as they get a certificate by the local sheriff’s office. Well, that date is rapidly approaching. AR-15 owners have until December 27 to certify their weapons. If owners fail to do this, they won’t be allowed to own their rifles within city limits. So far, only 85 rifles have been certified.