up here in canada, we have a firearms registry. contrary to popular misconception, guns aren't impossible to buy here - they're just very regulated. what that means is that the cops have a system they can check to see if you have a registered gun.
in fact, they are required to check the system and take special precautions. if the system tells them you have a gun, they have to behave differently. this is particularly useful for things like domestic violence complaints.
drawing a parallel, the tailgate scenario is not a "might create problems" issue, so much as a "will create problems" issue because there will undoubtedly be certain procedures that the police are required to follow should they come into contact with people on this list. and, perhaps, there should be - if the list is reasonably constructed, rather than dragneted.
worse is that any possible positive consequences will be quickly nullified by the system's lack of practical utility. it won't take long for the cops to realize that the list is far too broad to be useful. in some places, that won't matter - because the cops are racist and/or because there are very few minorities. in more liberal areas with more diverse populations - like seattle, say - it will quickly cease to be taken seriously. on the miniscule chance that a cop might actually be dealing with somebody worth detaining, they'll just ignore the database hit as yet another false positive.
i think the answer to this is changing america's foreign policy, but no change in foreign policy is going to completely eliminate the threat of crazy people doing crazy things. and, this is just badly designed as a means to address that.
i should point out that our dipshit conservative ruling party removed this registry a few years ago, but it was a very unpopular move and it should be one of the first things that's reinstated when they lose power.