To be pedantic: the 5.56x45 NATO round develops 1,177–1,708 J (868 - 1,260 ft-lbs) of energy at the muzzle, the 7.62x51 NATO round develops 3,275 J (2,416 ft-lbs) of energy. Factor in the weight of the firearm in question to offset that, but the .308 has a lot more recoil than a .223.