Mr. XXXXXXXXX
Address...
City, State Zip Dear Mr. XXXXXXXXX

This refers to your letter of August 18, 1995, in which you ask about the legality of a device known as the "Spud Gun." These subject devices are generally constructed from PVC tubing and fittings and are designed to launch a muzzle loaded potato using aerosol hair spray or other type of propellant. Ignition is by means of some type of "spark" igniter. The Bureau has previously examined devices known as "Spud Guns, Potato Guns, or Spudzookas" and have determined that such devices, in and of themselves, are not firearms as defined in Title 18 United States Code (U.S.C.), Chapter 44, S 921(a)(3) or 26 U.S.C., Chapter 53, S 5845. However, any similar devices which can be determined to be weapons by reason of their design construction, intended use, actual use, ammunition or other factors may meet the definition of a firearm under Title 18 or 26 U.S.C. We suggest that you contact your State and local law enforcement authorities concerning possession of such devices. We trust that the foregoing has been responsive to your inquiry. If we may be of further assistance please contact us.

Sincerely yours,
Edward M. Owen, Jr.
Chief, Firearms Technology Branch

So basically check with your local law enforcement/authorities on whether these are legal in your area. The laws & such vary incredibly from city to city, & state to state, and as well as very greatly from country to country. Just be smart with their use, & you should be okay. Just don't cast bait with them in [Australia].