The external dimensions of the two cartridges, the 308 and the 7.62x51 NATO round are supposed to be the same. The real difference twixt the two is the higher pressures of the 308 hunting cartridge. The 7.62x51mm NATO round averages around 47k psi chamber pressures while the 308 can go as high as 60k psi and some folk do not advocate the use of the higher pressure 308s in military rifles chambered for the 7.62x51mm NATO round. HTH. Mikey.