Okay the XM8 has three diffrent barrel lengths, 9 inch for PDW, 12.5 inches for basic assault, and 20 inches for designated marksman and LMG. Since most general infantry will be issued the 12.5 inch model will the fragmentation effects of the 5.56 round be lost (aka less lethal because no fragmentation) when using a XM8 (12.5 inch) versus an M4 (14.5 inch). It was my understanding that the 5.56 needed a minium barrel length of 14.5 inches to achieve fragmetation.

It just wont fragment out as far.  With M193 ammo, it wil still fragment out to around 50 yards.

I think when they issue the M-8 they are going to issue heavier ammo too (77gr). Just a hunch.