I have a friend that fought religious insurgents in Lebanon where he grew up...They had US supplied M16s and a few M4s. He, and some of my US military buddies, tell me many stories of 5.56 just making through and through holes in people. It slowed them down, but didn't always stop them.
He said he never had that issue with the FAL he used.
Much of that came from the military changing barrel lengths and twist rates but keeping the ammo designed for the old 20", 1/12 twist barrels. It caused the bullets to overstabilize. The Russians copied the 5.56 and kept in unstable so when it entered it would do backflips and cause more damage.