Mandatory gun buybacks aren't politically popular. The main reason for that is that there's a lot of gun owners.

Even if politically popular, the legal changes necessary to do so are very difficult and would be easily derailed by pro gun interests.

If you offered $500 a gun which is probably a fair average value, it would cost $200 billion dollars just to buy 400 million guns. That doesn't include the infrastructure to administer the buybacks and have the weapons destroyed. Let's say another 50 bil.

250 billion is a lot of money for a best case scenario in terms of implementation. But mandatory buybacks are likely to be extremely controversial and will need to use force against the non compliant.

The vast majority of gun owners are honest people that never used their gun improperly. Even so, a very small percentage of radicals among a population of approximately 100 million owners is still millions of people.

If they don't comply, do you send in police to confiscate? Would the police follow through and execute warrants en masse against people holed up at home with their guns? I certainly wouldn't.