Advantages of the .300 Blackout are can use .223 cases, common .308 diameter bullets bolts and magazines if using a modern sporting rifle(black rifle). 7.62x39 still has cheap surplus and bulk hornady 123 grain interlock for reloading. There aren't any .30 cal bullets I know that reliably perform if loaded subsonic so that advantage of the .300 is kind of negated to me.
Are suppresors legal down in Australia?
Form the cartridges you listed the 6.5 grendel and .223 are my favorites for differing reasons. With careful use the .223 is certainly capable for deer. You're using the .222 and 55 grain zmax bullets with success currently so you're familiar with the considerations needed. A properly set up .223 will do more than your current .222, better hunting bullets and launching high BC bullets to learn shooting skills to last a lifetime for cheap is one of them. The Grendel with careful use can be put to service taking larger game and has mild recoil and report you both may come to appreciate. Magazine length will really hamper the Grendels performance with slippery bullets but as a hunting gun I think its an often overlooked gem.
A .350 Legend may be your huckle berry I don't know about you're current availability but locally it has remained available and if not cheap reasonable in price.
Are suppresors legal down in Australia?
Form the cartridges you listed the 6.5 grendel and .223 are my favorites for differing reasons. With careful use the .223 is certainly capable for deer. You're using the .222 and 55 grain zmax bullets with success currently so you're familiar with the considerations needed. A properly set up .223 will do more than your current .222, better hunting bullets and launching high BC bullets to learn shooting skills to last a lifetime for cheap is one of them. The Grendel with careful use can be put to service taking larger game and has mild recoil and report you both may come to appreciate. Magazine length will really hamper the Grendels performance with slippery bullets but as a hunting gun I think its an often overlooked gem.
A .350 Legend may be your huckle berry I don't know about you're current availability but locally it has remained available and if not cheap reasonable in price.