The FAL prototypes were chambered for a FN propriatary 7mm short cased cartridge similar to but not identical to the British .280 short cased intermediate cartridge which an experimental bull pup Enfield assault rifle and LMG was chambered for.
The US 6mm cartridge that the Lee Straight pull rifle had been chambered for was the basis of experimental LMGs and it became obvious early on that a .30 or larger was better suited to the purpose.
The 7.62X51 ( a good ol metric designation if there ever was one) has never come up short in its intended role, so long as fed the proper ammo for whatever purpose it is put to. You can't say the same for the 6.5 cartridges used by the Japanese, Italians, and some others.
The 7mm Mauser is pretty much in the same class as the 7.62X51.
Every combatant of WW2 that started out with a 6.5 cartridge adopted a 7.7 or 7.35 to replace it, or fielded MGs in .30 or 8mm rather than use their own standard infantry cartridge for that purpose.
The only real objection to the 7.62X51 has been controlability in full auto fire from a lightweight rifle.
The Japanese got around this by issuing cartridges with a lighter load for use in selective fire rifles. The Spanish did much the same when they first fielded the CETME rifle, though the purpose was more to ease extraction till the fluted chamber was developed.