http://www.rcmp-grc.gc.ca/cfp-pcaf/faq/index-eng.htm#a3
Minimum barrel length is 470mm, or 18.504".
Minimum OAL must be 660mm, or 25.984".
So, for all intents and purposes, 18.5" and 26".
You CANNOT alter a barrel so that it is shorter than 18.5", regardless of action, unless you're a barrel manufacturer. Period.
A manufacturer can produce a shorter barrel that is NOT designed to be on a semi automatic (pump, lever, bolt, break, etc) action, and maintain NR status. A barrel shorter than 18.5" on a semi is restricted if its OAL is 26" or over, or prohibited if its OAL is shorter than 26".
Edit: ####, you're exactly right. Longer than 470mm/18.5" for semi NR, modified to shorter than 457mm/18" for prohibited.
That's EVEN MORE complicated than I thought... Fantastic, try to explain THAT to someone new, lol.