Right - the answer is no, although they're both so-called 'intermediate-powered', or 'assault-rifle' cartridges, they are quite different, with the .223 rem (or 5.56mm NATO or 5.56x45mm) round sending a 55-62 grain, .224 caliber bullet out at 3000 to 3100 fps, whereas the Russian 7.62x39mm sends a 123 grain, .311 caliber bullet out at about 2350 fps. The other Russian round, not to be confused with the first 2 mentioned here, is the one originally made for the AK74 rifle, and that is the round known as 5.45x39mm, which is essentially the same case as the 7.62x39mm, but necked down to take a .221 caliber bullet. It has ballistics similar to the .223, of around 50-55 gr bullets going out at around 2900 fps, IIRC.