Peanuts, in comparison. There are more than 300 million firearms in circulation in the U.S.

Australia was never a gun culture in the first place. Apart from security forces, civilian guns were things farmers used for stock control, or occasionally hunting. It used to be the same in the US as well, until the weekend warrior/home security people went berserk.

The real problem in the US is the cultural trope that 'problems are solved by a man with a gun'. From domestic police to foreign policy, the US psyche approves of an armed response early. Until that's sorted (which is a generational problem, with no 'quick fix before the next election'), the US is going to keep having this issue, again and again.

You'd also be destroying most of the $6 billion dollar gun industry and approximately 200,000 jobs in addition to all of the costs associated with trying to extract millions of guns from the populace.