Funny thing is, no one here has mentioned the fact that inorder for it be a firearm it must meet two criteria.
1. 500 FPS or more
2. Amount of Joules (not sure on the number).
If it doesnt meet both of those conditions its not a fire arm. Last time I checked, and I could be wrong, a 6mm .20 to .40 bbs couldnt reach the amount of Joules required until something like 700 FPS.
Just an outside point.
As a retailer of parts, dealing with the CBSA is a major time consumer. I cant tell you how many days I play telephone tag with the Prohibited Items Unit, hell I have them on Speed dail.