One idea is to have a bank service for automatic gun owners to bank their automatic weapons, where they can still immediately access them in times of needing to fight against a hostile and corrupt government (which doesn't happen all that often anyways), whilst not needing them everyday. They will still have their everyday single shot pistols to stop thieves and bank robbers and terrorists.
This can be done in a decentralised manner on a blockchain, merely requiring some threshold of consensus of agreement that indeed government needs to be fought. You would just want to limit the stake of any gun-holder's coins in a single bank (which are distributed when you hold the guns), otherwise the whales could easily reach the threshold themselves.
The black market would still exist, but for everyday gunowners, theyd probably be happy with this solution since it preserves the liberty of owning the weapons but also protects the liberty of school kids not having to worry about being mowed down by automatic weapons.
Build it America.
Your friend Australia.