What? Surely the collective expertise of the forum members is not baffled? ;-)
I’ve been looking through the Stevens/Ezell SPIW book, in which the cartridges are initially referred to as being of .22 calibre. However, the XM110 is referred to in one official document as 5.6 mm. Presumably the XM144 is the same, as this used the same flechette/sabot assembly. However, elsewhere in the book the XM110 and XM144 are officially referred to as 5.56 mm. Later, a specification table for the same rounds reverts to 5.6 mm.
For the second generation rounds, the XM645 is designated 5.6 mm in a couple of places in the book - yet as I posted I have seen a packet of XM645 labelled 5.8 mm.
So on balance, the most common designation seems to be 5.6 mm, but both 5.56 mm and 5.8 mm have also been used.
Can anyone add anything to this?