First, my understanding is that at least Colt and Olympic Arms both make their rifles with .311" bores, at least on guns made for the last 10 years or so. This may not have been true of earlier rifles, though. I know for certain that Ruger's Mini-30 has used .311" bores since at least the mid-80s.
Also, I'm fairly certain that US-made 7.62x39 has used .311" bullets for some time. Winchester, Remington, and Speer all list ~125gr .311 bullets for reloading as well. I've got some domestic ammo from Federal and Winchester in this caliber that I can test this weekend.
Some background:
The bores on both US and Russian "30 Caliber"-class guns starts at .300" before the rifling is cut, and this is also the . What is different is that, on US weapons, the rifling is cut right at .004" deep. Add the bore diameter plus the depth of the grooves on both sides of the bore (.300 + .004 + .004) and you get .308".
The Russians cut their rifling just a bit deeper, at around .0055, so the diameter between the grooves is about .311" Having deeper rifling allows the rifle to grip the bullet even with the barrel is heavily fouled.
The problem is that Russian and European ammo is loaded with bullets of .311" diameter, whereas most "30 Caliber" bullets in the US are .308" These bullets will work in the Russian guns, but may lose some velocity, have jacket scoring, and may not be as accurate in the larger .311" bore.
Firing .311" bullets in a .308" bore will result in increased chamber pressures and increased wear on the bore. This is exacerbated with steel-jacketed ammo, especially on non-chromed bores.
[b]The main problems with 7.62x39 ARs continue to be be the difficulty of getting the round to feed in the AR's straight mag well, and the severe lack of suitable hi-cap mags, which are increasingly more expensive and difficult to find.[/b]
-Troy