7.62 NATO is not the same as .308 Winchester (
mini FAQ). They are
esessentially interchangeable, but they
are different, and rated to different pressures (.308 being higher).
Nota Bene: same case with its little brother, as 5.56 NATO != .223 Remington (but the NATO spec is higher pressure).
This doesn't necessarily mean that you
don't have a pressure problem (to wit: the CBC caution in the mini FAQ); if that's the case it should be pretty obvious (mashed primers) and you can verify by firing a string and then removing the firing pin to see if it's any easier to open, but I'm with Grumulkin -- shoot what works and don't worry about it.