SPIW calibres?

Can anyone clarify the correct calibres and nominal calibre designations for the various SPIW flechette rounds?

They seem to be generally designated “5.6 mm” but I have seen a box of XM 645 labelled “5.8 mm”.

So - are the actual calibres all the same (and if so, how should they be designated), or did either the actual calibres, or the official designations, vary?

Any information gratefully received…

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?

In report AD-317299 the cartridge is officially named as “5.6 mm XM110 (Single Flechette)”. See also AD-316128, where simply .22 Flechette is used for naming the tested cartridge. (Search for “single flechette” on DTIC)

Added: from my notes it is obvious that in a BRL report the calibre of the XM645 was given as “5.8 mm”. This is an independent source from the SPIW book, but I do not have this report, and it is seemingly not available from DTIC.

FWIW, XM110, the only SPIW cartridge I have measures .222 at the base of the projectile micrometer jaws resting on case mouth, my 5.56 cartridges measure .223, my .222 rem - .220, 5.7 FN - .224 and so on. 5.45 has laquer that gets in the way. My AAI 5.56 with flechette also measures .223. I know this has nothing to do with the designation and everyone here knows this!

Tony, these are the designations I have found in labels, displays, official documents, manuals, etc. describing the four basic fléchette cartridges:

XM110: .22 or 5.6 mm
XM144: .22 or 5.6 mm
XM216: 5.6 mm
XM645: 5.77 or 5.8 mm



found on the internet: 5.8 et 5.77mm

Thank you gentlemen, even if some of the information adds to the confusion!

In the interests of simplicity I think I’ll stick to 5.6 mm for the XM110, XM144 and XM216, and 5.8 mm for the XM645.

In the book SPIW The Deadliest Weapon That Never Was, in the chapter entitled The Multiflechette Concept are the following items:

Winchester 9.53 multi-flechette cartridge, 4 flechettes, page 107 (9.53 x 76mm)

10 mm flechette cartridge, 5 flechettes, fig. 106, page 108

7.62 x 51mm flechette cartridge, 3 flechettes, fig. 107, page 109

Thanks for that, but I should have clarified that I only meant the little single-flechette rounds.