I don't see confiscation being a viable nationwide option. The amount of resources required would be well beyond what I believe we (as a nation) are capable of, especially when you think that it would not be wise to send just 1 LE person to each residence.
If retroactive, I would expect the law would require you to turn them in or risk a significant fine & or jail. Then, you'd be unable to use them at the range, be in trouble if you possessed one in a traffic stop or some other unrelated encounter with LE, etc. Basically they would work to purge them from being in the hands of citizens by intimidation and attrition.
"No matter how bad it gets, if you're still alive it's just another bad day."