Who would go and ask for the guns? How would they ask? What would the response be? These are all important questions. This is where the rubber meets the road when we talk about taking away guns that people already own.
IMO, the first question asked about any proposed law should be "is it fair?" The second question should be "Is it enforceable?" Right now people are all hot and bothered debating the fairness without anyone giving any thought to enforcement.
Actually, the FIRST question should be "Is it Constitutional?" The Second question should be "Will it solve the problem I'd proposing it to adress?"
In answer to your first point- I imagine that in writing your hypothetical law, that someone would be charged with enforcing it.
As to how they would ask, I envision first a polite registered letter ro all those who registered the subject firearms.
Something to the effect that; "Possibly you are not aware that your weapon serial number XXX is now illegal for you to possess. You must turn in such weapons by date XX/XX/XX to avoid significant civil and criminal penalties".
Sincerely,
Your gun confiscation agency.
And escalating from there. Next a very threatening letter. Maybe a couple more letters, then a demand to appear in court on a certain date to answer a criminal complaint, or a warrant would be issued.
Then a visit from a number of very heavily armed people, probably in the middle of the night, accompanied by door-busting and flash-bangs and dog-shooting.
Think of the Randy Weaver case, or any number of SWAT team "dynamic entry" types of warrant serving.