If you buy an AR15 today, and there is not a registration requirement in your state, there is not one nationally. There will be a paper trail from the manufacturer, to the FFL, to You. If at some point registration does become required, you would most likely then have to register it. An exemption for previously owned firearms is possible, but doesn't seem likely to me. Of course, I'm not a lawyer, but I think it would be a bit of a stretch to apply ex post facto to a registry.
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