I don't want judges to be the final answer on what is a gun and what isn't. Do you? Do you want an unelected judge ruling that a handgun is not protected from infringement since, by his definition, "arms" are limited to long guns (rifles and shotguns)?
Let the legislature define firearms. If the people who elected them to office disagree, they'll let them know.
My question was how far legislatures should be allowed to go in redefining words. IMHO, for an act to be criminal it must be forbidden both by a broad common-meaning reading of the act, and by any explicit definitions provided therein.
If the goal is for a piece of legislation to forbid someone from possessing a firearm, airgun, or other weapon that propels an object down a barrel using high-pressure gasses derived from any sort, the legislative text should use the term "projectile weapon" rather than firearm. The definition of "projectile weapon" could then be narrowed in further legislative text, but could not be widened to include non-projectile weapons. If the legislation was designed to address non-projectile weapons as well, then it should just use the term "weapons" or else designate them clearly: "It shall be unlawful for (person) to possess a Section-13 weapon. The term "section-13" weapon includes all devices which use high-pressure gasses, from any sort, to propel any solid object or collection of objects with a total momentum greater than XX or a total kinetic energy greater than YY.