If you are referring to the original Squad Automatic Weapon project from many years ago, I am not sure. The original 6mm SAW used the existing 5.56mm (.223 Rem) case.
The neck on the 6mm SAW sure looks a lot longer to me than the neck on the 5.56mm case.....
Do you have a link to a picture or anything. I actually have a Barrel and 6mm SAW ammo, but it is possible that there was an experimental round before the final one.
Official cartridge designation is 6X45mm
I see. If you note the 1st type (on the left) has a case about as long as the 5.56X45 but the shoulder is set back making the neck longer. This round, when loaded was too long to fit in m-16 mags and was dropped pretty early I think. Same with the Aluminum cased ammo that had a slightly longer case and an even longer O.A.L.
This is a wild guess but the round looks kind of similar to the 6.5 Grendel. I suspect with a 140 you would be getting around 2400 to 2500 from a 20". I think the Grendel is set up for a 120 (which is not really all that bad sectional density wise) at about 2700+ from a 20". A 140 6.5 is *very* long.
It certainly has some merit for long range stuff in a carbine. Tactical penetration is not bad either.