The U.S. Army completed a study of current 5.56mm M855 round, in response to complaints from troops that this ammunition was in adequate in combat. Troops reported manyinstances where enemy fighters were hit with one or more M855 rounds and kept coming. The study confirmed that this happened, and discovered why. If the M855 bullet hits slender people at the right angle, and does not hit a bone, it goes right through. That will do some soft tissue damage, but nothing immediately incapacitating. The study examined other military and commercial 5.56mm rounds and found that none of them did the job any better. The study concluded that, if troops aimed higher, and fired two shots, they would have a better chance of dropping people right away. The report recommended more weapons training for the troops, so they will be better able to put two 5.56mm bullets where they will do enough damage to stop oncoming enemy troops.
The study did not address complaints about long range shots (over 100 meters), or the need for ammo that is better a blasting through doors and walls. The army had been considering a switch of a larger (6.8mm) round, and the Special Forces has been testing such a round in the field. But a switch is apparently off the table at the moment. The U.S. Marine Corps is doing its own study, but has not finished it yet.
The army report is not likely to be well received by the troops.