In 1945, the United Kingdom was positively awash with guns. The authorities asked people to hand them in for destruction; many people did, but many did not.
Over the next few decades, police forces around the country had occasional amnesties, whereby citizens who had found the pistol or rifle dad had 'souvenired' during the war could hand them in, no questions asked.
Nevertheless, many guns from that period are still out there in the UK, and occasionally one is used in a crime, or found when clearing a deceased estate.
Simply declaring that you must either register or sell your guns to the government (as we did in Australia after the Port Arthur Massacre) would certainly make a massive impact - the idea that if you can't get every single unregistered gun then there is no point getting any of them is, frankly, insane.
But then, sanity and the US gun lobby are not really related concepts.
If you got just a 10% reduction in the number of guns in circulation, that would be a big positive. Do that a few times, and guns become uncommon. Do it a few more times, and guns become rare.
The idea that buy-backs, confiscations or amnesties cannot work because some people will keep their guns anyway is like the idea that it is impossible to walk from New York to Los Angeles, because you can't get there by sunset. You don't have to get there today; all you have to do is move slowly but relentlessly in the right direction.