Ever wondered about the difference between the .308 Win and the 7.62×51 cartridge?
Before we go much further, we want to address the oft-posed question “are the .308 Winchester and 7.62×51 NATO one and the same.” The simple answer is no. There are differences in chamber specs and maximum pressures. The SAMMI/CIP maximum pressure for the .308 Win cartridge is 62,000 psi, while the 7.62×51 max is 50,000 psi. Also, the headspace is slightly different. The .308 Win “Go Gauge” is 1.630″ vs. 1.635″ for the 7.62×51. The .308′s “No-Go” dimension is 1.634″ vs. 1.6405″ for a 7.62×51 “No Go” gauge. That said, it is normally fine to shoot quality 7.62×51 NATO ammo in a gun chambered for the .308 Winchester (though not all NATO ammo is identical). Clint McKee of Fulton Armory notes: “[N]obody makes 7.62mm (NATO) ammo that isn’t to the .308 ‘headspace’ dimension spec. So 7.62mm ammo fits nicely into .308 chambers, as a rule.” You CAN encounter problems going the other way, however. A commercial .308 Win round can exceed the max rated pressure for the 7.62×51. So, you should avoid putting full-power .308 Win rounds into military surplus rifles that have been designed for 50,000 psi max.
A UK armourer explains the problems he had using reloaded cases fired from military 7.62×51 rifles in his .308 rifle:
Around this period I discovered that shooting reloaded cases fired from an FN was virtually impossible. They didn’t want to chamber. My frustration led me to ask an older armourer what was wrong. In a nutshell, he told me that they probably didn’t fit because they stretched. I was using a Lee Loader in 308 Winchester and didn’t know that they only resized the neck, leaving the rest of the case untouched. The shoulders were blown forward on initial firing, so the case was simply too long to fit my rifle’s chamber! Hmmm…
Some more more information here: