I wish I had a dollar every time a what if post was made. I would be so rich, I think I could have honest to god bought a house or something.
The bottom line is that if the CBSA agent suspects anything, they will check your baggage, sometimes even if they don't suspect anything they could still randomly check your baggage. If your disassembled airsoft gun is undeclared and you get checked and it gets found, you better have a very well worded explanation as to what it is, why you have it and why you didn't declare it.
The possibility of a big pile of border crossing bull shit just to save a couple of bucks? Totally not worth it, especially if it were to cause the family grief and annoyance during a vacation.
You have all the required the information you need and the general consensus is not to do it, do so as you please but your actions are of your own accord.
"Its only a little bit on fire"