[NateKirk]

Personally, your plan would not bother me much. I only own one semi-auto, and for the type of shooting I do I wouldn't miss really miss it anyway as long as I have my bolt guns.

However,

There are many, many, many people who feel differently.

Firstly, an Australian style gun buyback shouldn't be used as a modal for the rest of the world; the US and Australia have different cultures and histories concerning firearms, and we couldn't necessarily expect the same results here.

Secondly, mass shootings are not really a major problem, just a news worthy problem. The number of people who die from drug overdoses, car accidents, etc, everyday is much greater than those who die from intentional attacks with firearms. It might sound callous to say, but a mass shooting where 10-60 people die is just a drop in the bucket compared to other everyday causes of death. According to the CDC more than 91 people a day die from opioid overdoses alone.
This is not to say that we shouldn't work toward the problem. More openness and acceptance for mental heath treatments, waiting periods, and better qualifications for gun ownership can all help the problem somewhat, but none can completely solve it. There are other detached ways of killing many people at once not involving guns.

Thirdly, a forced gun buyback would be expensive if the American public is to get their money's worth. Who'll determine the value of an individual firearm to be bought back? What do we do with all the guns afterward? Millions tens, or even hundreds of millions, semi automatic rifles, shotguns and handguns, equals a lot of money. Are we to rip off the American public by taking their property for far less than it's worth, or are we to spend the money and siphon it from other public works to balance the books?

I would go into the virtues of having a public at a fighting level with the powers that be, however since you stated you are not concerned with the government (now or in the future I assume) I wont argue the point. I will say that, in conclusion, I think a gun buyback like you propose is not worth the effort considering the problem.