The 5.45mmx39mm Soviet is probably the one I'd start from if we were going to go all new cartridge mode. Base it on that.
Umm, the Soviets went to the 5.45x39 from the 7.62x39, in order to gain the same perceived benefits of our 5.56. They stayed with the x39 so that they could use the same basic weapon design. Kalashnikov himself never approved of the change. He felt they got it right the first time.