The left would have you believe the Second Amendment was penned to supply a militia or army. The right would have you believe the amendment was for self-protection. Neither viewpoints are entirely correct. Although the U.S. Supreme court ruled the Second Amendment guarantees the right to possess firearms for protection, the ruling is only partly inclusive of the Amendments importance.
The United States exists only because the common people were able to carry out an armed revolution against their government. The Second Amendment was written to protect the will of the people against the government; when the right to press the government under the First Amendment fails, the Second was written to take our county back. We would not have the United States if people did not own and use firearms.
Many (I would say most) Americans would view government confiscation of firearms as a threat to the citizens ability to protect and police themselves. I doubt many Americans would voluntary hand their firearms in. For example, when California made it illegal to own standard capacity firearms magazines (limited to 10 rounds under new law) I think a total of zero people turned their standard capacity magazines in. Guns are part of the United States culture, unlike Australia, the citizens of the United States identify with their firearms.
Also a gun buyback would break the government. The United States has an estimated 90 million firearms in private ownership, compared to the 640,000 brought back to the Australian government, how could the government reimburse gun owners? Even if we short-change the public and only reimburse $300 per firearm, it would cost the taxpayers $27,000,000,000 not mentioning the extreme costs to run the buyback program.