Gregdownunder.
I saw pictures of the outcome of your ban I note your concern that semi auto rifles not handed in would enter the grey market and contribute to making society less safe I agree with you that it potentially will but what you describe is exactly the situation in America ie unlicensed semi auto centre fire rifles available to all.
If its any comfort the roughly estimated tens of thousands of semi automatic five shot shotguns that disappeared when by law they had to be converted to three shot dont seem to have emerged in criminal hands