A gun database enables mass confiscation, and historically a gun database precedes mass confiscation by a few short years.
Nazi Germany, initially using "well intentioned" Wiemar law, in fact (the period after the WWI defeat was very chaotic and bloody). See this book by the #1 RKBA legal scholar for all the details: https://www.amazon.com/dp/1598131613/
The description also helps confirm this next example:
Any country they invaded with registration lists, including the Netherlands; perhaps the #1 US RKBA activist of the 20th Century got started down that path in basic training after WWII, when a refugee from there related an atrocity he'd witnessed, where a whole family was machine gunned in the village/town square after the head of the household couldn't produce a handgun the register said he had.
The above examples are particularly instructive since by definition you can never know who might in the future use such data for ill.
Soviet Russia if I remember correctly; in fact, if I remember correctly, there was not a single genocide in the 20th Century that wasn't proceeded with the pattern, aside from China, where the Communists instead dried up the supply of guns held by the population by imposing "rifle taxes" on the wealthy. Basically, their life was forfeit unless they supplied the Communists with a certain number of guns, and they moved heaven and earth to do that, not that it did them any good in the end.
Of course, the poorer the population in general, the easier it is to pull of such a gambit, or a registration followed by a confiscation, so many fewer have guns.
Communist Cuba, for sure. They came up with a catchy slogan, got too many people to register, confiscated the guns, and then gangs of them started having fun with their class enemies.
As noted elsewhere in this discussion, in smaller numbers for select types of weapons, with of course the option to remove them from the state, this has happened in California when the status of the obsolescent (but certainly effective, e.g. used by the recent Dallas cop shooter) SKS, and in New York State. Something like it might be playing out right now in Massachusetts as their AG decided to redefine their "assault weapons" law, but she's still changing her not actually all that official position on it (i.e. it's published in unofficial venues like the office's web cite, no proper rule making process was followed).