...Simple non compliance by enough people would put the government in an unpleasant position to expand a police state, which wouldn’t be popular with the general public and would be extremely costly both politically and financially for the government to pursue....
The Canadian gun registration had so much non-compliance, they gave up, eventually.
Canada Tried Registering Long Guns -- And Gave Up
https://www.forbes.com/sites/danielfisher/2013/01/22/canada-tried-registering-long-guns-and-gave-up... Canada tried it and gave up, discovering like several other nations that attempting to identify every gun in the country is an expensive and ultimately unproductive exercise. Criminals, of course, don't register their guns. And even law-abiding citizens tend to ignore registration when it comes to long guns mostly used for hunting and target shooting....
...The program turned out to be far more expensive than expected and didn't have any discernable impact on crime, perhaps because long guns are used so rarely by criminals in the first place. Canada's gun homicide rate, according to the handy statistics at Gunpolicy.org, has held steady since the late 1990s.
Canada passed a strict gun-control law in 1995, partly in reaction to a 1989 shooting at Montreal’s Ecole Polytechnique with a semiautomatic rifle. The law required universal regulation of guns, including rifles and shotguns. Proponents said the central registry would give law-enforcement agencies a powerful new tool for tracking guns used in crimes. They also claimed it would help reduce domestic violence and suicide....
...In 2002 Canada's auditor general released a report saying initial cost estimates of $2 million (Canadian) had increased to $1 billion as the government tried to register the estimated 15 million guns owned by Canada's 34 million residents....
... From 1997 to 2005, only 13% of the guns used in homicides were registered. Police studies in Canada estimated that 2-16% of guns used in crimes were stolen from legal owners and thus potentially in the registry....
...gun registration rarely delivers the results proponents expect. In most countries the actual number registered settles out at about a sixth. Germany required registration during the Baader-Meinhof reign of terror in the 1970s, and recorded 3.2 million of the estimated 17 million guns in that country; England tried to register pump-action and semiautomatic shotguns in the 1980s, but only got about 50,000 of the estimated 300,000 such guns stored in homes around the country...