Hello Grizzly,
I assume from your post that you are under the age of 18, is that correct?
If Australian laws make it illegal for under 18s to buy replica guns then you would be commiting an offence doing so. If a store won't sell you one over the counter, you can be safe in the knowledge the store will protect themselves from prosecution by making you declare that you are over 18 on any Online Purchase systems. If you tell lies and are caught out, you could end up in Police custody.
Best advice is, if it's illegal, don't even think about trying it.
If you want to learn how to load firearms, the Denix models are a waste of time because it's not possible to chamber rounds of any description in them.
Perhaps you'd be better off waiting till you're old enough to join the Military. They'll teach you how to use as many firearms as you like properly and safely.
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.