What is a “mandatory†buy back, other than a seizure followed by “just compensation†under the takings clause?
Will the "just compensation" pay for the price of the firearm plus modifications, if any, made by the owner?
Highly unlikely. Typically, the money offered in these schemes is a fraction of the price of the firearm, deemed sufficient by some one who really isn't in any hurry to pay what the firearm is worth.
You can argue that the value declined when it was banned, but reality is that the value of things which have been banned goes up, not down, if those items are in demand (25 round magazines for a 10-22 rifle during the Clinton AWB were bringing multiples of their former market price, for example). There has been demand for firearms since their invention.