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....