I don't know. Does that make it MORE prohibitted?
If it is prohibitted for some other reason than barrel length and you shorten the barrel......hmmm.
Yes it does make it prohibited outside of the 12(5) class. Remember barrel length is specifically defined in the legislation and the law clearly states that if you modify the barrel of a semi-auto to below 18.5" it is a prohibited device and the firearm it is attached to is prescribed to be a prohibited firearm.
We have two different pieces of legislation that apply to the same firearm. One part applies to the firearm itself, that's where we get all the different classes and the the other part applies to the barrel, which allows them to move the same firearm within those classes. If the law only defined the firearm and not the barrel then it would be a non issue. So being a 12(3) or 12(5) is only a designation of a class of firearms just like restricted non-restricted, other laws still apply to the barrel length.
The rules regarding barrel length apply to firearms that are already prohibited. If you have an FN with a 16" barrel that is already classed as 12(5) prohibited and you cut the barrel to 14" that particular firearm is now prescribed as a prohibited firearm outside of the 12(5) class and is illegal. Section "(d) any firearm that is prescribed to be a prohibited firearm" alows them to prohibit your prohibited firearm. The "prescribed to be prohibited" part of the legislation allows the government a LOT of leeway.
There are lots of people and gunsmiths that have made the chop without knowing the rules but it is not legal. Simply having a 12(x) designation does not allow you carte blanche to do whatever you want to the firearm.
So be very carefull before you decide to start chopping!!!
If you need confirmation on any of this, simply call the CFC and ask to speak with a firearms technician, that's what I did and after two months of arguing this area with the CFC it was pretty clear that the laws were not in my favor.