Well, the determination of it being a pistol vs. a rifle would be something for the CFC to decide. If your tube is from a known 'rifle'- i.e. a Crosman 2260 or Discovery (the tube is what they CFC has used as the 'receiver' for classifying things in the past as it has the serial number on it)- then it should, by default, be classified as a rifle. The problem is that if it is classified a Restricted firearm, then it must also be registered- so if nothing else, double-check things so no one gets into trouble.

In any event, it looks good. What is joining the two tubes together (i.e for gas flow)?