User blog comment:AdvancedRookie/Activision hints at more frequent DLC and content variety/@comment-190.149.117.152-20120106010813/@comment-4103312-20120106203724

Still, the ACR is designed as an adaptive platform, while the AR-15 is designed as 5.56 mm weapon. And there would be a point, having the 5.56mm (weak and accurate), 6.8(increased damage and recoil) and 7.62 with the same attributes as the 6.8 with shorter range but even slightly higher (minimum) damage