Trying to squeeze more performance out of what is basically a varmint cartridge.If they want to double the effective range,they should start issuing 7.62x51 caliber weapons.
I think something in between using the same cartrige size, like the 6.5 and 6.8mm rounds out there would probably be best. I believe they were working on such a round and some special forces use it. For one thing, it would still use the same M4 receiver and magazine, just change out the upper. No need to retrain troops to a new rifle.
I guess if the effective range is doubled thye will now be qualifying at 600 instead of 300 meters? I like the 7.62 round a lot, but for most (non combat arms) soldiers the 5.56 rounds and rifles are lighter and easier to carry and shoot.