In Massachusetts, they already maintain a registry of all guns, including private transfers. I’m sure there are old guns that people have that pre-date the registry plus of course the illegally owned guns but most are in the registry at this point. In addition, every time a gun license holder moves, they must notify the local police of the town they are leaving, the local police of the town they are going to, and the State. Even if you move within a town, you have to notify the local police and the State. I had to notify the local police and the State when I moved to Vermont. Fortunately VT doesn’t do any of that stuff so there wouldn’t have been anyone up here looking for my records. Some of my gun owner friends in MA look wistfully at me up here. This is why my son will be moving to VT too. There are many common handguns that cannot be sold or owned in MA. The State maintains an approved list, the State Attorney General being the person who grants approvals. MA has a very low rate of gun ownership.