Neither air rifles nor air pistols are regulated as firearms, so yes, you can cut down the barrel of of an air rifle without violating any federal laws. As to local or state laws, you would need to check that out further. FYI, there are full-auto bb machine guns - these are also not regulated under NFA.
http://www.drozdmax.com/
Bottom line - you can do whatever the heck you want with an air gun (for now) as long as it only use air/compressed gas for a propellant.