well then you all better not forget the 30-06 Springfield used in several older rifles(WWII guys especially cause the M1 Garand, BAR, 30cal Browning and a few others that are lesser known like the Johnsonville) the 30-06 is the 7.62X63mm(can be confirmed in Janes Guns Recognition Guide).
I am all for an M4 type firing the 7.62X39mm, but it would have to have a modification done to the magazine because AK and M4 mags are not interchangeable. another thought is that since the AK-74 is used more than the AK-47 that the M4 could be chambered for the 5.45X39mm used in the AK-74(believe this is accurate, no time to confirm), which would place it as smaller round that the 223 remington(5.56X45mm to you non-hunters)