A .308" diameter bullet is in metric 7.62mm, while a .311" is in metric 7.70mm. Though it is true that the bullet stretches and the diameter shrinks slightly upon projection from cartridge, I still wouldn't risk using a slightly larger cartridge when the weapon was designed specifically for the former. And I would be even more critical against using a smaller cartridge. If it were possible to use a larger bullet, the casing would still have to be within almost exact dimensions to be properly chambered. So to use a .311, it would have to be a 7.70x39mm round to guarantee less chance of error.
Btw, In that metric designation, the suffix numbers after the "x" refer to the length of the casing/cartridge in which the bullet fits. Many firearms are chambered for the same caliber, however each requires a different standard in cartridge dimensions. Some with longer casings, others with shorter. However if you compare a 7.62x39 bullet with a 7.62x51 or 7.62x54r bullet, you will see the differences are none to miniscule whereas the actual bullet length may be longer or shorter by a millimeter however the diameter is exactly the same (in appearance). I've got pretty good eyes, and they seem as exact as I can tell, though I'm no Terminator so couldn't be 100% positive.