Canada Post "Solutions for Small Business" question

[Drache]

Is there any reason why I can't just box up a firearm and mail it in the usual way, provided I remove the bolt, package it securely, have a useless trigger lock, etc etc?

Short answer - No

Long Answer - Yes

If you don't care about insurance on damage or parcel loss there is nothing stopping you. But if you don't mail it to their standards they reserve the right to void any insurance you have paid for.