Ridiculous waste of money and low fruit, when there are hundreds of millions of guns to go.
The only plus-side to a gun-buy-back....is where you have a weapon with enough damage or issues that repairing is too costly and not worth the effort. That’s the only time that this makes any sense. You have to remember, back in the 1970s....a ton of cheap pistols and rifles started to flood the market. None of those are worth keeping or ever fixing.