Bullets work in the principle of kinetic energy, the shorter the range of impact the more lethal it is.
Say for example rifle X (5.56 mm caliber) has an effective range of 500 meters, in close combat if it hits its target in 50 meters or less then the damage would be greater in proportion to its caliber. An AK-47 firing 7.62x39 caliber rounds at 10 meters or less would have the same devastating effect of a .50 caliber weapon at full range.
That is why many armies prefer the 9mm pistol round in CQB. It is light weight, more rounds can be carried, the weapon is compact and easy to grip, most importantly the light weight round makes it possible to fire accurately at full auto - something not possible with 7.62 or even the 5.56 caliber rounds.