These are air guns with both a high muzzle velocity (greater than 152.4 meters or 500 feet per second) and a high muzzle energy (greater than 5.7 joules or 4.2 foot-pounds). The “muzzle velocity” is the speed of a projectile at the instant it leaves the muzzle of a gun, normally expressed in metres per second or feet per second. The “muzzle energy” is the energy of a projectile at the instant it leaves the muzzle of a gun, expressed in joules or foot-pounds. Air guns need to meet both standards to be classified as firearms for purposes of the Firearms Act.
http://www.cfc-cafc.gc.ca/info_for-r.../airguns_e.asp