Well, explain it to him/her like this:
In SEVENTY YEARS of gun registration in the entire country of Canada, only THREE crimes total have been solved with the use of registration data.
In about FIFTY YEARS of gun registration in the state of Hawaii (which has very closed /controlled borders, so if there's anywhere that registration *should* work, in theory, to help solve crimes, it should be there), there have been exactly ZERO crimes solved with the use of registration data.
So, weigh that infinitessimally small potential benefit of registration against the enormous *potential* harm of it being a stepping stone to disarmament, which it nearly always is (happened in California with .50 cals, IIRC - had to register them under penalty of jail, and then had to turn them in under penalty of jail, correct?), which amounts to a complete obliteration of a fundamental civil right, and you can see that the cost/benefit analysis is not on the side of registration.
Katrina was most certainly door to door confiscation, but it's not too terribly relevant to a discussion of registration, because they just went a-lookin' and a-askin' for guns - no registration database used for that (there's NOT any in La). Those folks should have said go pound sand when asked if they have guns, and THEN the lack of registration would have worked to allow them to retain, so that's a great argument for no registration, even in the absence of use of registration, which COULD HAVE made an illegal disarmament episode even worse that it was/could be. Unfortunately, too many N.O. area residents were honest with Mr. National Guardsman, and admitted to having guns, so the lack of registration was of limited usefulness. Unfortunately, I think that next time a massive illegal rights-trample occurs, ultimately it would be better for our rights and the nation itself if the tree of liberty is refreshed a bit, along with citizens engaging in wholesale misrepresentations to the illegal disarming force. And I don't say that lightly, as my brother was a member of one of the national guard units sent down there, and yes, I would have accepted his death as a necessary condition to maintaining liberty, however difficult that may have been.