its not exaclty a weak round, but its small and very light. not to mention the balistics of 5.56 vs 7.62x51 is dramatically different. i can see someone taking 2 7.62 to the torso and going down but if its 5.56 might need to use a few more. this kinda reminds me of the Philippine-American war of 1899-1902. with the US re-issuing .45 colt cause the .38 long colt did not have enough stopping power.
incidents like the 1905 case of Antonio Caspi, who was shot multiple times with .38 Long Colt rounds but remained a threat until struck with a carbine butt.
This led to the Army re-issuing the older .45 Colt Single Action Army revolvers as an emergency measure.
im not saying that 5.56 is not a good cartridge, just not as effective as 7.62: due it its small size and light weight. why we have 5.56 as a standard issue? the 7.62mm round offers greater range and stopping power, the U.S. military determined that the advantages of the 5.56mm round—specifically increased ammunition capacity, reduced recoil, and improved controllability—outweighed these benefits for the typical combat environment.