funny story - when the law first came out in 1994 restricting barrel length for non-restricted firearms, lots of smiths made a cottage industry of lathing down M1 carbine muzzles and welding or brazing on pieces of pipe to get to 18.5". A good many of the NR M1's out there are likely still like this and have just not been re-verified in the intervening years.
In the late 1990's, the RCMP lab decided to "re-interpret" the law and they sent notices to all the gunsmiths in Canada (registered businesses) saying it needed to be 18.5" of RIFLED barrel measured from the bolt face.
Some gunsmithing shops started making their extensions out of .357" chunks of barrel stub and continuing to apply M1 carbine extensions. The RCMP didn't say continuous rifling of continuous diameter, so technically they were following the guidelines. After a couple years, the lab got wise to this and updated their gunsmithing guidance to say the barrels had to be continuously rifled for the full length of the barrel, excepting the chamber, at the diameter of the calibre it was assembled to fire.
So all this to say, I still encounter "non-restricted" M1 carbines with GI barrels and pieces of old .357" barrel stub neatly silver soldered to the muzzle. They are done so well that without noticing the muzzle bore diameter, you would think they were made that way.
As I understand it, if the lab ever sees these guns, they change the classification to restricted and register the M1 carbine. I have no idea if their "interpretation" of the Act has been tested in court or not - I suspect not though, and that it's akin to their "guidance" on deactivations.
