I don't see anyone here taking this to court to challenge the system. You say it costs too much and rightfully so, but honestly if you challenge it for just your rifle and get this fight started it will help.
You can start by calling up the CFO and telling them that your 20" AR is restricted and you want to know why.
They will tell you it's restricted due to variation of the M16 due to the firearms act C-68.
You can then say that the firearms act says that the AR15 is a variant of the M16 and in reality this is wrong and that the M16 is a variant of the AR15 and not the other way around.
They will tell you that the law is the law, but this is your basis for your case in court.
Get a lawyer, explain the details, gather proof that shows that the AR15 was made first and that the M16 was spawned off of the civillian AR15. This will prove that the AR15 is not a variant of the M16 and is in fact only subject to the other rules of C-68 for a fireram. Barrel length.
I see guys on here spending 4000$ for an HK ar15, others spending 10K on other firearms. Personally if I had that kind of cash I'd spend it on my lawyer and bring it to court to have my own non-restricted AR15.
The facts are simple, we are in the right, no matter how much money the CFO has to represent themselves the facts are clear, the AR15 is not a variant of the M16 and thus falls under normal firearm regulations.