Ever notice how guns are always referred to as registered or not on “entertainment” TV? To the best of my knowledge only a few jurisdictions require gun registration: Hawaii, DC, NYC come to mind as the only jurisdictions where the local tyrants and their jackboots require registration.
I have noticed for 40 years, at least.
I do not know if it is part of a "style" book somewhere, or just a cultural thing, as the gatekeepers of screenwriters would not allow other nomenclature.
Robert A. Heinlein documented how he was forced to write-in gun registration into his novel Red Planet.