Originally, it was chambered for the 5.56. I know they were considering refitting it for a new cartridge, the 6.8 mm.
Don't know if they ever followed through with it though.
If you all would read Soldier of Fortune you would know already that not only is this weapon better than the M16 but that they have a 6.8mm top end that will drop right on top of the present M16 and the mags fit the magwell on the lower also. It will save the Feds mass money to drop these on the existing m16s 'til they have to change over.