Ah yeah, the ol' "it worked in Australia!" argument.
The Australian confiscation took out 660k of firearms in their "confiscation" program at an estimated $500MM in cost. That comes out to about $750 per gun. Keeping the math easy by rounding. There are 330
million firearms in the US currently. Let's just pretend that
half of those would be confiscated (silly, I know, but follow along). At the same buyback rate, that is 124
Billion dollars in a buyback program - not including the massive cost of managing such a program or administering it. Do you honestly think there is the political will anywhere to spend $124 Billion on such a program (which would probably be closer to double that with 2018 prices - remember Australia did this 15 years ago)?
Not to mention that this would still leave approximately 115 million firearms still in circulation - not counting the number that would spike once such a confiscation program was enacted? Or the guns that are illegally owned.
New York tried a buyback program and it fell flat on its face with approximately 4% of registered firearm owners adhering to the program. You're a European, and you don't understand - at all - the situation here in the United States, or the massive will that is against such an initiative.
Here's a good read on whether the assault weapons ban of 1994 (yes, we tried this before) actually worked and what effect it actually had.
https://www.factcheck.org/2013/02/did-the-1994-assault-weapons-ban-work/