Confiscation of unregistered weapons in a legal, responsible approach is something that would take some time. But it's also the only chance you have not to start a rebellion; if not civil war.
The only way I can see to do it, is over time. A big wave with all the weapons we know of up front; a smaller wave of "tax breaks or dollars" for unknown guns. Then just weed them out slowly as they are identified. When they're identified, owners get a trip to prison for, say a year, for each violating firearm they still have(tougher laws).
To me this is simply a gun ban. Weeding out bad guns over a couple decades.
We just have different views on what confiscation means and how it would play out. I do think a ban is 1,000 times more likely than gun confiscation.
Either way I think it leads to rebellion. The south, plains and mountain west would be inflamed over even a gun ban. ID, AZ, UT, MT, WY, ND, SD, TX, AR, OK, NE, KS, MO, AR, MS, TN, KY, AL, GA, LA, SC and WV would go nuts over that. Possibly other states would have real problems as well. Like AK, IA, ME, CA, IN and OH.