This is the number one question asked about spud gun amateurs. The answer is Of course they're legal!  It is stated so in the law.  The feds say they're OK but if you do anything such as shoot it over your school campus night, or shoot a car or pet (anything illegal) don't come to me if the police suspend it/you.  Please read my full disclaimer before you attempt anything.

It has been said many a time that spud guns are only as safe as the people shooting them.  Simply keep this in mind and you will be fine.