Originally Posted by Ronan
I'm pretty sure it has to be both, altho iv been told etheir one = firearm. but you will see pellet guns that are sold that have +500fps but under the joule speed and you don't need a firearm license.
That seems unlikely, seeing as how the smallest commonly-available caliber pellet/BB you'll find is .177cal (4.5mm), which are typically 7.9 grains (just a bit over 0.5 gram) -- posted velocities for .177 cal guns are usually based on that pellet weight.
Quick muzzle energy calculation yields 5.95j at the muzzle. (for 7.9gr @ 500fps)
Common .22 cal pellets are in usually in the neighborhood of 14 gr and are over the 5.7j limit well before 500 fps (360 fps for a 14.6gr pellet, to be precise).
I highly doubt the 5.7j limit was chosen arbitrarily.