6.5 is a good round,but IMHO,not as good as a 7 mm round because you can get heavier bullets in a 7mm. In fact,I think the 30 caliber round is the only round with wider bullet selection available,but 30 caliber rounds usually have too much recoil for the girl soldiers,they are heavier,and they are more expensive.
Then again,you can get a FN-FAL in 308 or 8 MM,also,and both are order boxes you could put a check mark in. Since you have to make a compromise and the prime reason the US Military wants to upgrade is because instead of swapping bullets with the enemy at 50 yards or less we are now swapping bullets across mountain valleys with swirling winds at 500 meters or more,the 7mm has a MUCH better bullet selection when it comes to the long,heavy bullets that buck the wind. Yeah,the 30 cal has heavier bullets for dangerous game,but you don't need 220 grain or heavier bullets to kill humans at a distance.
The 7x57mm is a compromise,but so are any other caliber you might mention,and I think it is the best compromise when you consider weight,expense, and effectiveness for expected use. Plus,you can pick up the phone and order a gazillion rounds of loaded ammo for it from any ammo manufacturer in the world and have them shipping out some the same day. The damn thing has been in production since the 1890's in one load or another.
AND.......just think of all the money the taxpayers can save by not having to pay for a new rifle design,new ammo load,and all the R&D that goes along with it.
Not to mention all the time and lives saved from having the new rifle and round you need TODAY,available TODAY.
The FN/FAL has been around forever it seems, and would be a good choice (would that be English or metric, sir?) I brought up the 6.5 X 55 because of the longer bullet, good ballistic coefficient, and range, which put it nearly on par with the .30-06 and 7.62 X 54 R, but lighter ammo, which the military wants. At the crossroads of impact energy, light ammo, long range, it seemed to fall pretty close to the paper. it doesn't kick as much as tho .30-06 or the 7.62 X 54. which is something else they apparently desire. (I guess shoulder bruises from pack straps and recoil just don't go with evening wear, and the ladies must be accommodated.)
Seriously, though, if our guys have any more to lug around with all that battle rattle, they'll need wheels on their asses. 100+ lbs of gear in the frigging desert? I won't badmouth anyone who can do that all day.
Recoil should be moot in any well designed modern battle rifle, simply because it will use some of that energy cycling the action, and it will be significantly reduced.
The M-14 is a decent rifle, imho, but my knockoff (M1-A) is in full USMC furniture, and far heavier with just a full box than the Stoner variants are dressed for the dance. At 13 lbs. plus ammo/magazines and optics, it is a lot for the average person to pack around, although that pales to the 'light' machine gun dad carried in Korea. The Garand action is reliable though, the rifle easy to clean, but an adjustable gas valve would be nice.
I haven't shot the FAL, and would like to, but the local guy who had them was known for hashing stuff together and I didn't trust anything he might have had a hand on. I'll leave that at that. Maybe someday. They definitely have a following, and, as you pointed out, are common among West bloc armies and have been for quite a while.
Like the BUFF, some basic designs just are difficult to improve on.
I noticed the Russians (Soviets) went for smaller when they re-did the AK to the -74, with the 5.45X39 round. Those have long bullets with great ballistic coefficient, are fast, and come 1000 to the spam can. Allegedly, the bullets behave very erratically on impact because of internal design, and were especially disliked by those downrange in Afghanistan when the USSR was there. But that's a smaller caliber, lighter bullet, and not what the Army wants.
Alternatively, the current Stoner variants can be re-done in virtually any caliber the military wants, with minor modifications. Unless the objective is to abandon the 'Mattel" altogether, which, unless the NFA is overturned, will likely end up in the hands of the police, given to some foreign country, or destroyed. They won't end up in civilian hands with the current trends in politics because the lowers are set up for select fire, and uppers (bolt carriers) could be adapted to work with that by anyone with a little know-how.
As far as price point, efficiency, and getting the job done, I think you are right, something proven, off-the-shelf, and readily available is cost efficient. But there is almost no way the US military will sign off on a foreign design for our troops' main battle rifle.
Instead, they'll reinvent the wheel and send it to the field to 'get the bugs out'. In the end, it seems like someone in ordnance in chasing their tail on this (or someone is going to make a grundle when the contracts are let).
The proven rounds have been around for a century or so because they work. Discovering yet another for general applications could happen, but why?