Regardless of your personal opinion of Australia's buy back program, it worked very well. The US used to have similar
programs, though I haven't seen any in several years. No reason we can't start them up again.
Laws against large capacity magazines, etc. - yes, we had those kind of laws for many years and they worked very well. Back in the 30's they were banned outright. No one outside a war zone needs high capacity magazines, nor armor piercing bullets. We have a military and law enforcement agencies who handle killing large numbers of people on our behalf.
Limiting the amount and type of ammo someone can buy is also prudent. I favor a heavy tax on it, as well as registration.
Can you provide an example in the US of registration resulting in confiscation of something? You have to register and license your automobiles and no one can impound them away unless you broke the law. Consequences, etc.
Armour piercing bullets are intended to kill police officers and others in law enforcement. They were banned for many years for that reason.