If you have a spud cannon that's capable of greater than 500fps, and you take it apart, are left with a few lengths of pipe, one of them is the receiver. But it's just a simple pipe.
Another issue - how do you lock out one of these guns?
In the case of mine, you take away the 120 psi compressor. Then it's useless.
The same could be said with an actual gun - put the magazine in a different room and it's useless. Still a receiver, still falls under the FA.
Are you getting this, pupil? You'll give your professor a headache with a paper on Canadian gun laws lol
The compressor is the receiver. The ammunition is air and anything that fits in the barrel. Which leads to the complication of 'how do I store it separately from the ammunition?'. It's a big barrel, not much doesn't fit in there....