User blog comment:3RDRANGER/Feedback On Battlefield 3 Beta/@comment-96.229.73.21-20111001194620/@comment-28584497-20111002043050

Er, the AK-74 and its derivatives (including the AK-74M, AKS-74U, RPK-74M) use the smaller 5.45mm x 39mm round.

NATO weaponry (including the M4, M16, HK416, M249) uses the 5.56 x 45mm round. The accuracy and power of the two are similar.