Not talking about the inserts, where talking about the size of the bore.
American .308 bores are .308'ish from the bottom of the grooves to the other bottom of the other groove (not the lands). Imported bores like the 303, 7.62 X 39 and 7.62 x54R have bores from groove to groove of .312'ish.
When a .308 bullet is fired down a .312 bore, the gas pressure is blown by since the bullet does not seal off the grooves (read working barrel pressures are lessened). On the other hand, when .312 bullets are fired down a .308 bore, the bullet has to slug it's self down from the normal .312 to the .308 size, and addition working barrel pressure is created.
Simple put, if you fire a .312 round that creates 50,000 psi in a .312 bore, through a bore of .308, the working pressure or the round is going to go up no less than 15,000, and you are now shooting loads at proof levels.
Again, there are two different types of bores in the 7.62 range. One is the .308 diameter, the second is the .312 diameter. Due to this, 7.62 bullets are grouped into these two ranges, and each group should not be confused with the other.
Regarding Wolf ammo, some of the 7.62 X39 ammo is actual loaded with the .308 bullets instead of the .312. The reasoning behind this is that the ammo can be shoot threw both bores without out causing excessive working pressures.