There are 270 Million firearms out there now (I had previously heard over 300 million, but I'll go with the low number for its own sake). That's a helluva lot of inventory.
Let's suppose that tomorrow, all new gun sales and manufacture was banned. What about our government's history in banning "things" makes you think that we'd have any success in actually banning the possession of firearms? They can be kept up and used, and resold, and absent invasive searches, would be pretty difficult for the government to find and detect. It really would take a tyrannical government to effect the prevention.
And again, even if all new sales and manufacture were banned, the law would be especially helpless in preventing home manufacturing of the type enabled by 3D printing. In the end, our choices are going to be "learn to live with weapon ownership" or "accept an invasive and tyrannical government."
I really can't see another way around it. Ridding the country of guns isn't worth tyranny to me. Let me be even more blunt. 30,000 lives per year isn't worth tyranny to me.