Actually under 500 fps is still classed as a "firearm"!
Under 500 fps are still firearms but have an exemption from requiring they be registered.
500 fps & over = needs to be registered!

There is so many things not true with the above
Is a PAL required under 500fps? No!! Only for firearms
Actually it is basically true. I did a little quick research for you and if you go to
http://www.rcmp-grc.gc.ca/cfp-pcaf/faq/index-eng.htm#a4
You'll see in the frequently asked questions FAQ's.........
" A4. As set out in the Firearms Act, "firearm" means:
•a barrelled weapon from which any shot, bullet or other projectile can be discharged and that is capable of causing serious bodily injury or death to a person, and includes
•any frame or receiver of such a barrelled weapon, as well as
•anything that can be adapted for use as a firearm.
Following are some weapons and devices that meet the definition of a firearm but that are deemed not to be firearms for purposes of the Firearms Act and related offences in the Criminal Code. Some of these items are
exempted from the definition only if they were designed exclusively for a specific purpose and are intended to be used exclusively for that purpose by the person who possesses it. However,
all of the items listed below are considered to be firearms if used in a criminal or negligent manner.
•Antique firearms;
•Devices designed exclusively for signalling, for notifying of distress, for firing blank cartridges or for firing stud cartridges, explosive-driven rivets or other industrial projectiles;
•Shooting devices designed exclusively forslaughtering domestic animals, tranquilizing animals, or discharging projectiles with lines attached to them;
•Air guns and other barreled weapons designed to have:
◦A muzzle velocity of 152.4 meters per second or less and/or
◦A muzzle energy of 5.7 joules or less."
So ......... if used in a criminal or negligent manner ..... a pellet pistol/rifle with less than 500 fps IS considered a firearm! That means if there's an accident resulting in injury (or most any other circumstance where police are likely to become involved) ...... you're into the Firearms Act involving the Feds & the criminal code!

So yes essentially less than 500 fps have an exemption from being registered but if anything happens, they ARE treated as firearms!! The devil is in the details and the sticky phrase is "criminal or negligent manner" which basically is a catch all for ..... THEY CAN CHARGE YOU! When it really matters, confronted by LEO's, chances are it will most likely be classified/treated as a firearm unless you happen to get a VERY understanding LEO!
EDIT: I am not a lawyer & this is my opinion only. Conduct yourself according to your own conscience & interpretation of exhisting policy. I believe my position is a prudent one when serving self preservation interests.