There is no mention of "dual calibre" so you're right... it is a made up term and may or may not hold up in court depending on the judge. However, it is this made up rule that is prohibiting us from getting an IIC for them from foreign affairs.
It is this same set of rules that they are applying to the 25rd 10/22 rifle magazines now... no official laws passed, just an arbitrary decision that made a bunch of people into criminals without them even doing anything.