I wondered how many guns that were deemed illegal in Oz were turned in with the buy back. Interesting stats. I also wonder how many guns were out there to start with. I can't find that stat:
Unlike the voluntary buybacks in the United States, Australian gun buybacks of 1996 and 2003 were compulsory, compensated surrenders of newly-illegal firearms.
The 1996 Buyback took 600,000 newly illegal sporting firearms, including all semi-automatic rifles including .22 rimfires, semi-automatic shotguns and pump-action shotguns.
Because the Australian Constitution prevents the taking of property without just compensation the Federal Government decided to put a 1% levy on income tax for one year to finance the compensation. The buyback was predicted to cost A$500 million and had wide community support
http://en.wikipedia.org/wiki/Gun_buy-back_schemeSo, the US has 300 million guns right now. I don't think a buy back would work, compulsory or otherwise. First of all, the citizens would bitch and moan about a tax, the tax would have to be much higher in the US and it would be virtually impossible to "police" the one's not turned in.
The "cold, dead hands" mentality would result in some shoot outs, imo. Americans are nuts about their guns, their right to those guns and would quickly organize into militia-type groups in order to fight the "evil government take over". IOW, they'd have the first part of the second amendment covered, finally (except for the well regulated part...

)