Just to let it be said, the Business FRT states, under the "HK MR" rifle in .223Rem, .308win, 5.56mm, and 7.62NATO (You read that right, there is only an "MR" rifle in the FRT, MR223 and MR308 are classed as variants of the "MR" rifle, NOT AR-15's), that the legal classification is "Restricted", under the legal authority of "CC 84(1) "restricted firearm" para. (b)", specifically:
"restricted firearm" means
(b) a firearm that
(i) is not a prohibited firearm,
(ii) has a barrel less than 470 mm in length, and
(iii) is capable of discharging centre-fire ammunition in a semi-automatic manner,
Therefore you COULD, IN THEORY, add a 18.5" barrel, and have it verified as non-restricted.