Yes, absolutely. Anything can be used as a weapon, and an air gun and a pellet gun are not exempt from that. Under the definition of a weapon in the Criminal Code, I could throw my coffee cup at you with intent and be charged with assault with a weapon. The same could be said for any of the pens that are on the table before you. It's no different with a BB or pellet gun. As long as it's being used in a responsible manner, there's no problem, but if you want to use it to commit a crime or in the commission of a crime, then yes, you are still open to criminal charges.
On May 14th, 2015. See this statement in context.