They mainly enforce it by charging you with a crime if they find that you own a gun but not a gun license. I'm pretty sure that the guns that the inner city gangsters in Massachusetts use are purchased legally in Vermont and New Hampshire (where licensing is more lax).

The cops don't look for guns, but if they find one, they'll arrest you and figure out if you own it legally later.

I can tell you would be appalled by the interview process I went through to get this card.