Before they can confiscate they have to know who has them. The best way to go about it is to require a federal license to own and use any firearm. It would be easy to get but would require everyone to list all the firearms in their possession. Stiff penalties would exist for violators. Once compliance has been established with the law abiding they can begin banning firearms incrementally according to type (using every tragedy as further justification). Owners will have a time limit to turn in the firearms or be criminally prosecuted for failing to comply. This way they don't need to confiscate the guns - americans will turn them in "voluntarily". In the end only the gov't and 'criminals' will possess them.