You don't need prisons for all. Just a few, have a lot moved (for safety) to evacuation centers. There is word that plans have already been made for these. The ones left with guns outside won't be able to practice, carry, or use them without a LOT of difficulty.
Why do you think the Homeland Security folks bought 450 million rounds of .40? Even Social Security bought something like 174,000. This is plus all the 5.56 and 7.62; all soft point, so it is not for overseas military.
Remember, it is "reasonable" to restrict handguns (or hicaps).