OK, the real problem facing any solution to the US gun problem is getting the guns off the streets. Yes, there will have to be a buyback which will be fun given that it will have to be on the same level as the Aussie one. There will probably be later gun amnesties, but without the monetary incentive.
So, cash in early.
There is a solution for those who can't wait to get your guns off the streets but aren't altruistic enough to just give them to someone without some kind of benefit.
The National Center for Unwanted Firearms
They say they are willing to buy or allow for donation of the firearms. That is a pretty good deal if one can realise the fair market value of your firearm: even if it's only as a tax deduction.
Gun control won't work in the US as long as there are guns in circulation which is why getting them off the streets is the most important aspect for dealing with this problem.