It was due to a restriction after WWII in NSW on ownership of military calibres. Not sure if any other states imposed the law.
No so much 30-30s but the classic one was 7.7x54 which I think was shortening the chamber by taking up the standard barrel by 1 turn, which got them around the law.
And of course it spawned all the rifles using the military actions but fitted with barrels chambered in 303/25, 303/270, 303/22?.
I think only graziers and pest controllers could still own a rifle in 303B.
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