Matt_243: You're right about the 5.56, it is not the best of choices for an effective battle round. My understanding of the Russian 5.45x39 is that it is a much more effective round in that the bullet weight is heavier, in the 60+ grain range and its construction, fmj with an internal hollow nose point, adds to the effectiveness.
The 7.62 x39 AK47 round is my choice over the 5.56x45. The 7.62x51 nato round is better yet but needs a better battle rifle that is handier for the troops to use.
Something like the 6.8mm they are talking about, or the 260 Remington (also a 6.5 mm) might be just the ticket.
However, if someone designs a 6.5x45mm round, which might be a shortened 260 Remington, you could replace the uppers of the M16s and continue to use the same platform. I agree with the opinion on what those Diggers experienced and would opt for a heavier round. Just my 2 cents worth though. Mikey.