You can potentially ban them immediately (realistically: you can't; it's not politically expedient), but you can't remove them immediately.

I think Australia removed them about as fast as anywhere did ever in a calm orderly way, after a mass-shooting that horrified the nation. Gun ownership is down quite a bit, but it's not a magic bull...uh...it's not magic: https://www.sydney.edu.au/news-opinion/news/2021/04/28/new-gun-ownership-figures-revealed-25-years-on-from-port-arthur.html

And while gun homicides and suicides are down (https://www.gunpolicy.org/firearms/compareyears/10/total_number_of_gun_deaths), homicides overall stayed flat even while gun homicides were declining (https://www.macrotrends.net/countries/AUS/australia/murder-homicide-rate).

So I think the immediacy point needs greater consideration.