The legality of the potato-firing spud gun varies among jurisdictions. As Of 1/11/2015 over 175 countries have legalized the Spud gun.
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United States
Spud guns are not federally regulated, although the ATF offers a classification. Legislation varies widely by state, county, and township. One would have to check with local authorities to determine local spud gun legality.
Australia
All combustion and pneumatic spud guns are considered firearms in every state. Any device capable of being aimed and discharging a projectile using a barrel, and that has the potential to cause injury to a person, is considered to be a weapon and requires licensing.
New Zealand
Pneumatic spud guns are classed as air guns in New Zealand, which means the owner must be either over 18, or over 16 with a A-Category Arms License. Combustion cannons are deemed a firearm, requiring the owner to hold an A-Category License. These laws are rarely, if ever applied and spud gun ownership and building has proved a popular hobby with many New Zealanders.
The Netherlands
All combustion spud guns are considered firearms.
United Kingdom
Pneumatic spud guns with projectile energy greater than 4 joules (3 ft·lbf) are classified as section 1 firearms and do require a license. In recent years, with the rise of spud gun use, there has often been much debate as to whether or not spud guns should actually be classed as Light Air Weapons. If spud guns were to be reclassified, then their maximum lawful projectile energy would be 16.3 joules not 4.
Germany
In Germany, a spud gun does not differ legally from any other firearm (for ones that use combustion to propel the projectile) or air rifle. The manufacture, acquisition and possession is therefor subject to the same requirements as for any other weapon of the same category. Since there is no known model developed before 1871, which would classify that model as an antique, spud guns are not subject to the eased regulations regarding antique weapons. Therefore any non-combusting spud gun with a maximum projectile energy of 7.5 joules can be owned by anyone without a license, while their manufacture still requires one. The legal possession of any other such weapon requires a gun license.
Canada
A spud gun is not considered a firearm unless it fires a projectile faster than 152.4 meters per second and at a muzzle energy exceeding 5.7 Joules based on the definition of a firearm in the Canadian Criminal Code. A spud gun may be interpreted as an imitation firearm, and therefore it would be illegal to have it in possession in a public place. The spud gun would most likely be considered a dangerous weapon, and therefore if an offense is committed with it, a possession of a dangerous weapon charge could be issued. If the air powered spud gun has soft ammunition (potatoes) that splatter or bounce on impact and is kept under 100psi the spud gun is perfectly legal.
Poland
Pneumatic spud guns are subject to the same laws as air guns, i.e. they are not classified as weapons if their muzzle energy is below 17 joules. Combustion spud guns are considered firearms.