Canada tried a long gun registry a few years ago. Non-compliance was somewhere north of 90%. In Canada.
They estimated total cost to be about $100,000,000 to institute the program. After wasting $2,000,000,000 they canceled the entire thing.
I heard about one old guy. Every single morning, he appeared outside his police statin and raised an old .22 rifle in the air and challenged the police to come take it. They never did.
Haha!