Re: the long range advantage of 7.62x51
Last time I looked, the Army studies of what distances combat occured at showed something like 90% of engagements began at less than 100m and 99% began at less than 300m. Now keep in mind, this is full-blown combat with support weapons and much more capable sensors (thermal sights on vehicles for example) involved.
Add in urban settings with shorter sightlines and more restrictive ROE, and even in combat, engagement ranges are likely to be well within the effective zone of either 5.56 or 7.62.
That leaves us with two advantages to 7.62x51:
1. Better barrier penetration
2. Better terminal ballistics
Except that M80 ball doesn't have better terminal ballistics than M855 or M193. It actually has pretty poor ballistics compared to those rounds, though if the military was able to use similarly constructed ammo in 7.62x51, then 7.62 would unquestionably have better ballistics.
So now we are down to:
1. Better barrier penetration
Which like many tools is a double-edged sword. The better penetration of 7.62x51 is great when you need it (stopping a vehicle at a check point for example) but not so great when you don't (fighting inside a urban structure with other friendlys present).
In addition, going to 7.62x51 gives up a third of your onboard ammo capacity and overall ammo capacity, as well as increases recoil. If you go down to the nice handy barrel sizes that 5.56 is popular in (14.5"/16"), you also get a lot of blast and muzzle flash in .308.
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If you insist on a short barreled rifle, than you need a new caliber.
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No, you just need a round loaded to optimize the shorter barrel. Check out "5.56mm SOST". One reason the military has been reluctant to go through the huge logistical nightmare of a caliber change is that companies like Federal/ATK have developed 5.56 rounds that can produce wound cavities in gel similar to 6.8. Though it does make you wonder what the same technology in a 7.62x51 would look like
