And at the same time:
"(b) a firearm that is adapted from a rifle or shotgun, whether by sawing, cutting or any other alteration, and that, as so adapted,
(i) is less than 660 mm in length, or
(ii) is 660 mm or greater in length and has a barrel less than 457 mm in length,"
DOESN'T make any exceptions for firearms that are ALREADY restricted or prohibited; by a straightforward reading of the law (and, remember, it doesn't HAVE to make any sense), that means if you take an AR-15 with a 16" barrel, and you cut that 16" barrel back to 14" (or any other length less than 457mm), you're creating a "prohibited firearm". What's it worth to you in legal fees to find out for sure?