[Xringshooter]

Different parts of the law books (US and PA) define it as a firearm, other parts do not. It depends on what that section is used for

Websters New Dictionary and Thesaurus define a firearm as: any hand weapon from which a shot is fired by explosive force, as a rifle or pistol.

The average person uses the above definition because they typically are not getting into the nuances of the law which comes up when some entity (police, BATFE, etc.) thinks that someone has broken the law.