Seems like this discussion goes in big circles - covering threads from at least a couple years ago. Your bolded section "any other alteration" - to your knowledge has there ever been a Canadian judge who had to interpret what that means?? The preceding words - "sawing, cutting" seem clear enough, but does "replacing" a barrel equal "any other alteration", and has any judge ruled on that one way or the other?
I suppose the issue becomes "alteration" to the firearm, versus "alteration" to that barrel? But, as mentioned, we need to know if a Judge or Judges have said that or not...
I have looked extensively and I can not locate a single judicial ruling that addresses "any other alteration". I have read hundreds of judicial reasons and I can say that the answer will depend largely on the judge, what they had for breakfast, and whether or not they have handled a complex firearms case before. And even then its a crap shoot.
I feel comfortable making a good speculation as to how a judge will attempt to answer the question, and the reasonable outcome of that process, but thats admittedly just my guess.
I'll spare you the long version, here's the short version.
Judge, what does this law mean?
Q.1. What was parliament trying to do when it made this law?
A. Prohibit the possession of firearms which have been modified by people to make them shorter, and thus more concealable.
Q.2. What did parliament actually prohibit (ie was exactly does the law say.
A. Edited for brevity...
Prohibited Firearm means a FIREARM that is adapted from a rifle or SHOTGUN, whether by sawing, cutting or any other alteration, and as adapted is less than 660mm OR has a barrel less than 457mm.
Q3. Does removing and replacing the barrel fall within the meaning of "any other alteration".
So to decide what any other alteration means, first, its adaptation of a FIREARM by XYZ. The law does not specify whether the adaptation is of the barrel only or if it includes any adaptation to the firearm as a whole, but a plain reading of the words and ordinary sentence structure suggests altering the barrel or the firearm as a whole would count.
Is changing the barrel an 'alteration'? Alteration is not defined in statute so we go to the dictionary. Take your pick.
Miriam Webster
1: the act or process of altering something
: the state of being altered
2: the result of changing or altering something: such as
a: a change made in fitting a garment
b : a change in a legal instrument that alters its legal effect
To Alter:
1. To make different without changing it into something else.
A. You could pick that apart for days and contrive all kinds of different applications, but its reasonable to conclude that switching the barrels is an alteration.
Q4. Does the alteration result in the firearm being shorter than 660mm or the barrel being 473mm.
A. Obviously yes.
Q5. Bonus question. Does this interpretation reasonable fall within what parliament intended to prohibit? Parliament intended to prohibit shortening firearms which makes them more concealable.
A. Any judge could go either way here, but its at least reasonable to assume some judges would readily agree that this fits. ANY OTHER MODIFICATION was included by parliament to capture ANY method of shortening a firearm that has the same result as cutting or sawing.
In summary, no judge that I am aware of has ruled on this. Your guess is as good as mine. Its reasonable to conclude that at least some judges would decide that swapping factory barrels is no different than sawing or cutting, when reading the definition of prohibited firearm.
The RCMP don't seem to really care, and as we have seen they typically ignore legal inconveniences when it suits them, but are also prone to changing their mind without warning.
Bringing it back to the OPs question, unless the firearm has a telescoping stock, there is no concern that a pump shotgun with a short barrel would be restricted. The question is whether it is NR or Prohibited.