Classifications of airsoft:
Toy (same as a nerf gun): Transparent receiver & shoots less than 366FPS
Uncontrolled firearm (same as pellet gun / antique): Shoots at least 366FPS but less than 500FPS (or energy less than 5.7 joules)
Prohibited Device/Replica Firearm: Non-transparent receiver & shoots less than 366FPS
Non-restricted Firearm: Shoots over 500FPS and energy over 5.7 joules and does not meet prohibited characteristics (fully automatic, barrel length for semi, etc.)
Restricted Firearm: Shoots over 500FPS and energy over 5.7 joules and does not meet prohibited characteristics (fully automatic, barrel length for semi and pistols, etc.) with overall length of 660mm or less (or is a pistol)
Prohibited Firearm: Shoots over 500FPS and energy over 5.7 joules and meet a prohibited characteristics (fully automatic, barrel length for semi and pistols, etc.)
Your best bet would be an 80% lower instead ;P