No damage is done by shooting .311's or .312's through a .308" groove diameter barrel. In my friend's rifle, the neck area is cut with the .303 reamer, thus there is lots of room for the neck to expand. You might not be able to take a normal .308" rifle chambered for say, a .308 or .30/06 and chamber it with a round loaded with a .312" bullet. The bullet might be too large to allow sufficient expansion of the neck to release the bullet, thus excessive pressure could result.
: In Keith's rifle's case, there is lots of room and thus no tight-neck syndrome(which will cause problems if present). Due to the mid range to slower powders used that develope maximum pressure some distance down the bore, ie: 6" to 12" ahead of the chamber, the bullet is already sized properly and thus does not increase breech pressure a measurable amount. Parker O. Ackley proved this many years ago, using IMR reclaimed 4895 powder and an '06 case and barrel with up to .358" bullets, appropriately necked and throated, of course. The bullets are sized, swaged if you will, in the angle of the throat to properly fit. There is not much difference between .308" and .312" bullets - a mere .002" per side.