The idea of a “class 5 weapon” comes from the movie “The Purge.” It is a purely fictional concept, and there simply is no such thing as a “class 5 weapon.” But people keep looking for them, logically thinking some authenticity went into elements of the film. It is likely the term was invented to sound authentic, while suggesting a world where new categories of weapons had been created.
The ATF does define four classes of weapons – Class I are imported rifles, handguns and shotguns. Class II are rifles, pistols, shotguns and silencers made in the USA. Class III weapons are things like machine guns, sawed off shotguns and short barrel rifles made anywhere in the world. Class IV weapons are things like destructive devices, grenades and similar weapons.